soil

Soil is a major component of the Earth's ecosystem. The world's ecosystems are impacted in far-reaching ways by the processes carried out in the soil, with effects ranging from ozone depletion and global warming to rainforest destruction and water pollution. With respect to Earth's carbon cycle, soil acts as an important carbon reservoir,{{cite journal |last1=Amelung |first1=Wulf |last2=Bossio |first2=Deborah |last3=De Vries |first3=Wim |last4=Kögel-Knabner |first4=Ingrid|author4-link=Ingrid Kögel-Knabner |last5=Lehmann |first5=Johannes |last6=Amundson |first6=Ronald |last7=Bol |first7=Roland |last8=Collins |first8=Chris |last9=Lal |first9=Rattan |last10=Leifeld |first10=Jens |last11=Minasny |first11=Buniman |last12=Pan |first12=Gen-Xing |last13=Paustian |first13=Keith |last14=Rumpel |first14=Cornelia |last15=Sanderman |first15=Jonathan |last16=Van Groeningen |first16=Jan Willem |last17=Mooney |first17=Siân |last18=Van Wesemael |first18=Bas |last19=Wander |first19=Michelle |last20=Chabbi |first20=Abad |date=27 October 2020 |title=Towards a global-scale soil climate mitigation strategy |journal=Nature Communications |language=en |volume=11 |issue=1 |pages=5427 |doi=10.1038/s41467-020-18887-7 |pmid=33110065 |pmc=7591914 |bibcode=2020NatCo..11.5427A |issn=2041-1723 |url=https://www.nature.com/articles/s41467-020-18887-7.pdf |access-date=29 December 2024 |doi-access=free }} and it is potentially one of the most reactive to human disturbance{{cite journal |last1=Pouyat |first1=Richard |last2=Groffman |first2=Peter |last3=Yesilonis |first3=Ian |last4= Hernandez |first4=Luis |journal=Environmental Pollution |volume=116 |issue=Supplement 1 |title=Soil carbon pools and fluxes in urban ecosystems |url=https://www.researchgate.net/publication/11526697 |date=February 2002 |pages=S107–S118 |doi=10.1016/S0269-7491(01)00263-9 |pmid=11833898 |access-date=29 December 2024 |quote=Our analysis of pedon data from several disturbed soil profiles suggests that physical disturbances and anthropogenic inputs of various materials (direct effects) can greatly alter the amount of C stored in these human "made" soils. }} and climate change.{{cite journal |last1=Davidson |first1=Eric A. |last2=Janssens |first2=Ivan A. |journal=Nature |volume=440 |title=Temperature sensitivity of soil carbon decomposition and feedbacks to climate change |year=2006 |issue=9 March 2006 |pages=165‒73 |url=https://www.nature.com/articles/nature04514.pdf |doi=10.1038/nature04514 |pmid=16525463 |bibcode=2006Natur.440..165D |s2cid=4404915 |access-date=29 December 2024 |doi-access=free }} As the planet warms, it has been predicted that soils will add carbon dioxide to the atmosphere due to increased biological activity at higher temperatures, a positive feedback (amplification).{{cite journal |last=Powlson |first=David |journal=Nature |volume=433 |title=Will soil amplify climate change? |year=2005 |issue=20 January 2005 |pages=204‒05 |url=https://climatescience.ru/uploads/pubs/0/0f/0fa/0fa597c6991df9576bcad69f4f3113f8.pdf |doi=10.1038/433204a |pmid=15662396 |bibcode=2005Natur.433..204P |s2cid=35007042 |access-date=29 December 2024 |archive-date=22 September 2022 |archive-url=https://web.archive.org/web/20220922110017/https://fr.art1lib.org/book/10543301/528a68 |url-status=live }} This prediction has, however, been questioned on consideration of more recent knowledge on soil carbon turnover.{{cite journal |last1=Bradford |first1=Mark A. |last2=Wieder |first2=William R. |last3=Bonan |first3=Gordon B. |last4=Fierer |first4=Noah |last5=Raymond |first5=Peter A. |last6=Crowther |first6=Thomas W. |journal=Nature Climate Change |volume=6 |title=Managing uncertainty in soil carbon feedbacks to climate change |url=https://www.crowtherlab.com/wp-content/uploads/2019/07/bradford2016NCC.pdf |year=2016 |issue=27 July 2016 |pages=751–758 |doi=10.1038/nclimate3071 |access-date=29 December 2024 |bibcode=2016NatCC...6..751B |s2cid=43955196 |archive-date=10 April 2017 |archive-url=https://web.archive.org/web/20170410025316/http://fiererlab.org/wp-content/uploads/2014/09/Bradford_etal_2016_NCC.pdf |url-status=live }}

Soil acts as an engineering medium, a habitat for soil organisms, a recycling system for nutrients and organic wastes, a regulator of water quality, a modifier of atmospheric composition, and a medium for plant growth, making it a critically important provider of ecosystem services.{{cite journal |last1=Dominati |first1=Estelle |last2=Patterson |first2=Murray |last3=Mackay |first3=Alec |journal=Ecological Economics |volume=69 |issue=9 |title=A framework for classifying and quantifying the natural capital and ecosystem services of soils |year=2010 |url=https://www.researchgate.net/publication/223852147 |pages=1858‒68 |doi=10.1016/j.ecolecon.2010.05.002 |bibcode=2010EcoEc..69.1858D |access-date=29 December 2024 |archive-url=https://web.archive.org/web/20170808082847/http://esanalysis.colmex.mx/Sorted%20Papers/2010/2010%20NZL%20-3F%20Phys.pdf |archive-date=8 August 2017 |url-status=live }} Since soil has a tremendous range of available niches and habitats, it contains a prominent part of the Earth's genetic diversity. A gram of soil can contain billions of organisms, belonging to thousands of species, mostly microbial and largely still unexplored.{{cite journal |last=Dykhuizen |first=Daniel E. |journal=Antonie van Leeuwenhoek |volume=73 |issue=1 |title=Santa Rosalia revisited: why are there so many species of bacteria? |year=1998 |url=https://www.researchgate.net/publication/13682480 |pages=25‒33 |doi=10.1023/A:1000665216662 |pmid=9602276 |s2cid=17779069 |access-date=29 December 2024 }}{{cite journal |last1=Torsvik |first1=Vigdis |last2=Øvreås |first2=Lise |journal=Current Opinion in Microbiology |volume=5 |issue=3 |title=Microbial diversity and function in soil: from genes to ecosystems |year=2002 |pages=240‒45 |url=https://www.academia.edu/13038690 |doi=10.1016/S1369-5274(02)00324-7 |pmid=12057676 |access-date=29 December 2024 }} Soil has a mean prokaryotic density of roughly 10⁸ organisms per gram,{{cite journal |last1=Raynaud |first1=Xavier |last2=Nunan |first2=Naoise |journal=PLOS ONE |volume=9 |issue=1 |title=Spatial ecology of bacteria at the microscale in soil |year=2014 |page=e87217 |doi=10.1371/journal.pone.0087217 |pmid=24489873 |pmc=3905020 |bibcode=2014PLoSO...987217R |doi-access=free }} whereas the ocean has no more than 10⁷ prokaryotic organisms per milliliter (gram) of seawater.{{cite journal |last1=Whitman |first1=William B. |last2=Coleman |first2=David C. |last3=Wiebe |first3=William J. |journal=Proceedings of the National Academy of Sciences of the USA |volume=95 |issue=12 |title=Prokaryotes: the unseen majority |year=1998 |pages=6578‒83 |doi=10.1073/pnas.95.12.6578 |pmid=9618454 |pmc=33863 |bibcode=1998PNAS...95.6578W |doi-access=free }} Organic carbon held in soil is eventually returned to the atmosphere through the process of respiration carried out by heterotrophic organisms, but a substantial part is retained in the soil in the form of soil organic matter; tillage usually increases the rate of soil respiration, leading to the depletion of soil organic matter.{{cite journal |last1=Schlesinger |first1=William H. |last2=Andrews |first2=Jeffrey A. |journal=Biogeochemistry |volume=48 |issue=1 |title=Soil respiration and the global carbon cycle |year=2000 |url=https://www.researchgate.net/publication/51997678 |pages=7‒20 |doi=10.1023/A:1006247623877 |bibcode=2000Biogc..48....7S |s2cid=94252768 |access-date=29 December 2024 }} Since plant roots need oxygen, aeration is an important characteristic of soil. This ventilation can be accomplished via networks of interconnected soil pores, which also absorb and hold rainwater making it readily available for uptake by plants. Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, the water-holding capacity of soils is vital for plant survival.{{cite journal |last1=Denmead |first1=Owen Thomas |last2=Shaw |first2=Robert Harold |journal=Agronomy Journal |volume=54 |issue=5 |title=Availability of soil water to plants as affected by soil moisture content and meteorological conditions |year=1962 |url=https://www.researchgate.net/publication/250098028 |pages=385‒90 |doi=10.2134/agronj1962.00021962005400050005x |bibcode=1962AgrJ...54..385D |access-date=29 December 2024 }}

Soils can effectively remove impurities,{{cite journal |last1=House |first1=Christopher H. |last2=Bergmann |first2=Ben A. |last3=Stomp |first3=Anne-Marie |last4=Frederick |first4=Douglas J. |journal=Ecological Engineering |volume=12 |issue=1–2 |title=Combining constructed wetlands and aquatic and soil filters for reclamation and reuse of water |year=1999 |url=https://www.researchgate.net/publication/222464331 |pages=27–38 |doi=10.1016/S0925-8574(98)00052-4 |bibcode=1999EcEng..12...27H |access-date=29 December 2024 }} kill disease agents,{{cite journal |last1=Van Bruggen |first1=Ariena H.C. |last2=Semenov |first2=Alexander M. |journal=Applied Soil Ecology |volume=15 |issue=1 |title=In search of biological indicators for soil health and disease suppression |year=2000 |url=https://www.researchgate.net/publication/222520930 |pages=13–24 |doi=10.1016/S0929-1393(00)00068-8 |bibcode=2000AppSE..15...13V |access-date=29 December 2024 }} and degrade contaminants, this latter property being called natural attenuation.{{cite web |url=https://semspub.epa.gov/work/HQ/401611.pdf |title=Community guide to monitored natural attenuation |access-date=29 December 2024 }} Typically, soils maintain a net absorption of oxygen and methane and undergo a net release of carbon dioxide and nitrous oxide.{{cite journal |last1=Linn |first1=Daniel Myron |last2=Doran |first2=John W. |journal=Soil Science Society of America Journal |volume=48 |issue=6 |title=Effect of water-filled pore space on carbon dioxide and nitrous oxide production in tilled and nontilled soils |year=1984 |url=https://fr.1lib.sk/book/55339558/7bc22a |pages=1267–72 |doi=10.2136/sssaj1984.03615995004800060013x |access-date=29 December 2024 |bibcode=1984SSASJ..48.1267L |archive-date=18 March 2023 |archive-url=https://web.archive.org/web/20230318043457/https://fr.art1lib.org/book/23108771/821c3f |url-status=live }} Soils offer plants physical support, air, water, temperature moderation, nutrients, and protection from toxins.{{cite book |last1=Gregory |first1=Peter J. |last2=Nortcliff |first2=Stephen |date=2013 |title=Soil conditions and plant growth |isbn=9781405197700 |publisher=Wiley-Blackwell |location=Hoboken, New Jersey |url=https://fr.1lib.sk/book/2156095/a3577f |access-date=29 December 2024 |archive-date=22 April 2023 |archive-url=https://web.archive.org/web/20230422182643/https://fr.book4you.org/book/2156095/fd863f |url-status=live }} Soils provide readily available nutrients to plants and animals by converting dead organic matter into various nutrient forms.{{cite book |last1=Bot |first1=Alexandra |last2=Benites |first2=José |date=2005 |title=The importance of soil organic matter: key to drought-resistant soil and sustained food and production |isbn=978-92-5-105366-9 |publisher=Food and Agriculture Organization of the United Nations |location=Rome |url=http://www.fao.org/3/a-a0100e.pdf |access-date=29 December 2024 }}

File:Estructura-suelo.jpg, a notation firstly coined by Vasily Dokuchaev (1846–1903), the father of pedology. Here, A is the topsoil; B is a regolith; C is a saprolite (a less-weathered regolith); the bottom-most layer represents the bedrock.]]

{{Pie chart

|caption = Components of a silt loam soil by percent volume

|value1 = 25

|label1 = Water

|color1 = blue

|value2 = 25

|label2 = Gases

|color2 = cyan

|value3 = 18

|label3 = Sand

|color3 = yellow

|value4 = 18

|label4 = Silt

|color4 = brown

|value5 = 9

|label5 = Clay

|color5 = grey

|value6 = 5

|label6 = Organic matter

|color6 = black

}}

A typical soil is about 50% solids (45% mineral and 5% organic matter), and 50% voids (or pores) of which half is occupied by water and half by gas.{{cite web |last=McClellan |first=Tai |title=Soil composition |url=https://www.ctahr.hawaii.edu/mauisoil/a_comp.aspx |publisher=University of Hawaiʻi at Mānoa, College of Tropical Agriculture and Human Resources |access-date=5 January 2025 }} The percent soil mineral and organic content can be treated as a constant (in the short term), while the percent soil water and gas content is considered highly variable whereby a rise in one is simultaneously balanced by a reduction in the other.{{cite book |last=Zhang |first=Hailin |title=Master Gardener's Manual |chapter-url=https://extension.okstate.edu/fact-sheets/print-publications/e/master-gardeners-handbook-e-1034.pdf |chapter=Soils and fertilizer |pages=54–63 |publisher=Oklahoma Cooperative Extension, Service Division of Agricultural Sciences and Natural Resources, Oklahoma State University |location=Stillwater, Oklahoma |access-date=5 January 2025 }} The pore space allows for the infiltration and movement of air and water, both of which are critical for life existing in soil.{{cite journal |last=Vannier |first=Guy |journal=Biology and Fertility of Soils |volume=3 |issue=1 |title=The porosphere as an ecological medium emphasized in Professor Ghilarov's work on soil animal adaptations |year=1987 |url=https://fr.1lib.sk/book/37554227/660688 |pages=39–44 |doi=10.1007/BF00260577 |bibcode=1987BioFS...3...39V |s2cid=297400 |access-date=5 January 2025 }} Compaction, a common problem with soils, reduces this space, preventing air and water from reaching plant roots and soil organisms.{{cite journal |last1=Torbert |first1=H. Allen |last2=Wood |first2=Wes |journal=Communications in Soil Science and Plant Analysis |volume=23 |issue=11 |title=Effect of soil compaction and water-filled pore space on soil microbial activity and N losses |year=1992 |url=https://www.researchgate.net/publication/240546132 |pages=1321‒31 |doi=10.1080/00103629209368668 |bibcode=1992CSSPA..23.1321T |access-date=5 January 2025 }}

Given sufficient time, an undifferentiated soil will evolve a soil profile that consists of two or more layers, referred to as soil horizons. These differ in one or more properties such as in their texture, structure, density, porosity, consistency, temperature, color, and reactivity. The horizons differ greatly in thickness and generally lack sharp boundaries; their development is dependent on the type of parent material, the processes that modify those parent materials, and the soil-forming factors that influence those processes. The biological influences on soil properties are strongest near the surface, while the geochemical influences on soil properties increase with depth. Mature soil profiles typically include three basic master horizons: A, B, and C. The solum normally includes the A and B horizons. The living component of the soil is largely confined to the solum, and is generally more prominent in the A horizon.{{sfn|Simonson|1957|p=17}} It has been suggested that the pedon, a column of soil extending vertically from the surface to the underlying parent material and large enough to show the characteristics of all its horizons, could be subdivided in the humipedon (the living part, where most soil organisms are dwelling, corresponding to the humus form), the copedon (in intermediary position, where most weathering of minerals takes place) and the lithopedon (in contact with the subsoil).{{cite journal |last1=Zanella |first1=Augusto |last2=Katzensteiner |first2=Klaus |last3=Ponge |first3=Jean-François |last4=Jabiol |first4=Bernard |last5=Sartori |first5=Giacomo |last6=Kolb |first6=Eckart |last7=Le Bayon |first7=Renée-Claire |last8=Aubert |first8=Michaël |last9=Ascher-Jenull |first9=Judith |last10=Englisch |first10=Michael |last11=Hager |first11=Herbert |title=TerrHum: an iOS App for classifying terrestrial humipedons and some considerations about soil classification |journal=Soil Science Society of America Journal |date=June 2019 |volume=83 |issue=S1 |pages=S42–S48 |doi=10.2136/sssaj2018.07.0279 |s2cid=197555747 |url=https://www.researchgate.net/publication/332080061 |access-date=5 January 2025 }}

The soil texture is determined by the relative proportions of the individual particles of sand, silt, and clay that make up the soil. File:SoilTextureTriangle.svg plot is a visual representation of the proportions of sand, silt, and clay in a soil sample.]] The interaction of the individual mineral particles with organic matter, water, gases via biotic and abiotic processes causes those particles to flocculate (stick together) to form aggregates or peds.{{cite journal |last1=Bronick |first1=Carol J. |last2=Lal |first2=Ratan |title=Soil structure and management: a review |journal=Geoderma |date=January 2005 |volume=124 |issue=1–2 |pages=3–22 |doi=10.1016/j.geoderma.2004.03.005 |url=https://www.academia.edu/72307009 |access-date=5 January 2025 |bibcode=2005Geode.124....3B }} Where these aggregates can be identified, a soil can be said to be developed, and can be described further in terms of color, porosity, consistency, reaction (acidity), etc.

Water is a critical agent in soil development due to its involvement in the dissolution, precipitation, erosion, transport, and deposition of the materials of which a soil is composed.{{cite web |url=https://www.fao.org/3/r4082e/r4082e03.htm |title=Soil and water |website=Food and Agriculture Organization of the United Nations |access-date=12 January 2025 }} The mixture of water and dissolved or suspended materials that occupy the soil pore space is called the soil solution. Since soil water is never pure water, but contains hundreds of dissolved organic and mineral substances, it may be more accurately called the soil solution. Water is central to the dissolution, precipitation and leaching of minerals from the soil profile. Finally, water affects the type of vegetation that grows in a soil, which in turn affects the development of the soil, a complex feedback which is exemplified in the dynamics of banded vegetation patterns in semi-arid regions.{{cite journal |last1=Valentin |first1=Christian |last2=d'Herbès |first2=Jean-Marc |last3=Poesen |first3=Jean |journal=Catena |volume=37 |issue=1 |title=Soil and water components of banded vegetation patterns |year=1999 |url=https://www.academia.edu/35300713 |pages=1‒24 |doi=10.1016/S0341-8162(99)00053-3 |bibcode=1999Caten..37....1V |access-date=12 January 2025 }}

Soils supply plants with nutrients, most of which are held in place by particles of clay and organic matter (colloids){{cite book |last1=Brady |first1=Nyle C. |last2=Weil |first2=Ray R. |date=2007 |chapter=The colloidal fraction: seat of soil chemical and physical activity |title=The nature and properties of soils |pages=310–357 |edition=14th |editor-last1=Brady |editor-first1=Nyle C. |editor-last2=Weil |editor-first2=Ray R. |publisher=Pearson |location=London, United Kingdom |isbn=978-0132279383 |chapter-url=https://www.researchgate.net/publication/309630422 |access-date=12 January 2025 }} The nutrients may be adsorbed on clay mineral surfaces, bound within clay minerals (absorbed), or bound within organic compounds as part of the living organisms or dead soil organic matter. These bound nutrients interact with soil water to buffer the soil solution composition (attenuate changes in the soil solution) as soils wet up or dry out, as plants take up nutrients, as salts are leached, or as acids or alkalis are added.{{cite web |url=http://eagri.org/eagri50/SSAC121/lec14.pdf |title=Soil colloids: properties, nature, types and significance |website=Tamil Nadu Agricultural University |access-date=12 January 2025 }}

Plant nutrient availability is affected by soil pH, which is a measure of the hydrogen ion activity in the soil solution. Soil pH is a function of many soil forming factors, and is generally lower (more acidic) where weathering is more advanced.{{cite web |url=https://www.researchgate.net/publication/305775103 |last=Miller |first=Jarrod Ottis |title=Soil pH affects nutrient availability |access-date=12 January 2025 }}

Most plant nutrients, with the exception of nitrogen, originate from the minerals that make up the soil parent material. Some nitrogen originates from rain as dilute nitric acid and ammonia,{{cite journal |last1=Goulding |first1=Keith W.T. |last2=Bailey |first2=Neal J. |last3=Bradbury |first3=Nicola J. |last4=Hargreaves |first4=Patrick |last5=Howe |first5=M.T. |last6=Murphy |first6=Daniel V. |last7=Poulton |first7=Paul R. |last8=Willison |first8=Toby W. |journal=New Phytologist |volume=139 |issue=1 |title=Nitrogen deposition and its contribution to nitrogen cycling and associated soil processes |year=1998 |pages=49‒58 |doi=10.1046/j.1469-8137.1998.00182.x |doi-access=free |bibcode=1998NewPh.139...49G }} but most of the nitrogen is available in soils as a result of nitrogen fixation by bacteria. Once in the soil-plant system, most nutrients are recycled through living organisms, plant and microbial residues (soil organic matter), mineral-bound forms, and the soil solution. Both living soil organisms (microbes, animals and plant roots) and soil organic matter are of critical importance to this recycling, and thereby to soil formation and soil fertility.{{cite book |last=Kononova |first=M.M. |date=1966 |title=Soil organic matter: its nature, its role in soil formation and in soil fertility |edition=2nd |publisher=Elsevier |location=Amsterdam, the Netherlands |isbn=978-1-4831-8568-2 |url=https://fr.1lib.sk/book/2275488/56c210 |access-date=19 January 2025 |archive-date=22 March 2023 |archive-url=https://web.archive.org/web/20230322091500/https://fr1lib.org/book/2275488/ea4395 |url-status=live }} Microbial soil enzymes may release nutrients from minerals or organic matter for use by plants and other microorganisms, sequester (incorporate) them into living cells, or cause their loss from the soil by volatilisation (loss to the atmosphere as gases) or leaching.{{cite journal |last1=Burns |first1=Richards G. |last2=DeForest |first2=Jared L. |last3=Marxsen |first3=Jürgen |last4=Sinsabaugh |first4=Robert L. |last5=Stromberger |first5=Mary E. |last6=Wallenstein |first6=Matthew D. |last7=Weintraub |first7=Michael N. |last8=Zoppini |first8=Annamaria |journal=Soil Biology and Biochemistry |volume=58 |title=Soil enzymes in a changing environment: current knowledge and future directions |year=2013 |pages=216‒34 |doi=10.1016/j.soilbio.2012.11.009 |bibcode=2013SBiBi..58..216B |url=https://www.academia.edu/25235991 |access-date=19 January 2025 }}

{{main|Soil formation}}

{{Further|Soil mechanics#Genesis}}

Soil is said to be formed when organic matter has accumulated and colloids are washed downward, leaving deposits of clay, humus, iron oxide, carbonate, and gypsum, producing a distinct layer called the B horizon. This is a somewhat arbitrary definition as mixtures of sand, silt, clay and humus will support biological and agricultural activity before that time.{{cite journal |last1=Sengupta |first1=Aditi |last2=Kushwaha |first2=Priyanka |last3=Jim |first3=Antonia |last4=Troch |first4=Peter A. |last5=Maier |first5=Raina |date=2020 |title=New soil, old plants, and ubiquitous microbes: evaluating the potential of incipient basaltic soil to support native plant growth and influence belowground soil microbial community composition |journal=Sustainability |volume=12 |issue=10 |pages=4209 |doi=10.3390/su12104209 |doi-access=free |bibcode=2020Sust...12.4209S }} These constituents are moved from one level to another by water and animal activity. As a result, layers (horizons) form in the soil profile. The alteration and movement of materials within a soil causes the formation of distinctive soil horizons. However, more recent definitions of soil embrace soils without any organic matter, such as those regoliths that formed on Mars{{cite journal |last1=Bishop |first1=Janice L. |last2=Murchie |first2=Scott L. |last3=Pieters |first3=Carlé L. |last4=Zent |first4=Aaron P. |date=2002 |title=A model for formation of dust, soil, and rock coatings on Mars: physical and chemical processes on the Martian surface |journal=Journal of Geophysical Research |volume=107 |issue=E11 |pages=7-1–7-17 |doi=10.1029/2001JE001581 |bibcode=2002JGRE..107.5097B |doi-access=free }} and analogous conditions in planet Earth deserts.{{cite journal |last1=Navarro-González |first1=Rafael |last2=Rainey |first2=Fred A. |last3=Molina |first3=Paola |last4=Bagaley |first4=Danielle R. |last5=Hollen |first5=Becky J. |last6=de la Rosa |first6=José |last7=Small |first7=Alanna M. |last8=Quinn |first8=Richard C. |last9=Grunthaner |first9=Frank J. |last10=Cáceres |first10=Luis |last11=Gomez-Silva |first11=Benito |last12=McKay |first12=Christopher P. |date=2003 |title=Mars-like soils in the Atacama desert, Chile, and the dry limit of microbial life |journal=Science |volume=302 |issue=5647 |pages=1018–1021 |doi=10.1126/science.1089143 |pmid=14605363 |url=https://www.researchgate.net/publication/9020258 |access-date=19 January 2025 |bibcode=2003Sci...302.1018N |s2cid=18220447 }}

An example of the development of a soil would begin with the weathering of lava flow bedrock, which would produce the purely mineral-based parent material from which the soil texture forms. Soil development would proceed most rapidly from bare rock of recent flows in a warm climate, under heavy and frequent rainfall. Under such conditions, plants (in a first stage nitrogen-fixing lichens and cyanobacteria then epilithic higher plants) become established very quickly on basaltic lava, even though there is very little organic material.{{cite journal |last1=Guo |first1=Yong |last2=Fujimura |first2=Reiko |last3=Sato |first3=Yoshinori |last4=Suda |first4=Wataru |last5=Kim |first5=Seok-won |last6=Oshima |first6=Kenshiro |last7=Hattori |first7=Masahira |last8=Kamijo |first8=Takashi |last9=Narisawa |first9=Kazuhiko |last10=Ohta |first10=Hiroyuki |date=2014 |title=Characterization of early microbial communities on volcanic deposits along a vegetation gradient on the island of Miyake, Japan |journal=Microbes and Environments |volume=29 |issue=1 |pages=38–49 |doi=10.1264/jsme2.ME13142 |pmid=24463576 |pmc=4041228 |doi-access=free }} Basaltic minerals commonly weather relatively quickly, according to the Goldich dissolution series.{{cite journal |last=Goldich |first=Samuel S. |date=1938 |title=A study in rock-weathering |url=https://fr.1lib.sk/book/91942187/414bb2 |journal=The Journal of Geology |volume=46 |issue=1 |pages=17–58 |bibcode=1938JG.....46...17G |doi=10.1086/624619 |issn=0022-1376 |access-date=19 January 2025 |s2cid=128498195 |archive-date=27 March 2022 |archive-url=https://web.archive.org/web/20220327065200/https://fr.art1lib.org/book/60175497/a54b2b |url-status=live }} The plants are supported by the porous rock as it is filled with nutrient-bearing water that carries minerals dissolved from the rocks. Crevasses and pockets, local topography of the rocks, would hold fine materials and harbour plant roots. The developing plant roots are associated with mineral-weathering mycorrhizal fungi{{cite journal |last1=Van Schöll |first1=Laura |last2=Smits |first2=Mark M. |last3=Hoffland |first3=Ellis |date=2006 |title=Ectomycorrhizal weathering of the soil minerals muscovite and hornblende |journal=New Phytologist |volume=171 |issue=4 |pages=805–814 |doi=10.1111/j.1469-8137.2006.01790.x |pmid=16918551 |doi-access=free |bibcode=2006NewPh.171..805V }} that assist in breaking up the porous lava, and by these means organic matter and a finer mineral soil accumulate with time. Such initial stages of soil development have been described on volcanoes,{{cite journal |last1=Stretch |first1=Rachelle C. |last2=Viles |first2=Heather A. |year=2002 |title=The nature and rate of weathering by lichens on lava flows on Lanzarote |journal=Geomorphology |volume=47 |issue=1 |pages=87–94 |doi=10.1016/S0169-555X(02)00143-5 |bibcode=2002Geomo..47...87S |url=https://fr.1lib.sk/book/50018654/f73474 |access-date=19 January 2025 |archive-date=22 April 2023 |archive-url=https://web.archive.org/web/20230422182644/https://fr.art1lib.org/book/17831662/8253cd |url-status=live }} inselbergs,{{cite journal |last1=Dojani |first1=Stephanie |last2=Lakatos |first2=Michael |last3=Rascher |first3=Uwe |last4=Waneck |first4=Wolfgang |last5=Luettge |first5=Ulrich |last6=Büdel |first6=Burkhard |year=2007 |title=Nitrogen input by cyanobacterial biofilms of an inselberg into a tropical rainforest in French Guiana |journal=Flora |volume=202 |issue=7 |pages=521–529 |doi=10.1016/j.flora.2006.12.001 |bibcode=2007FMDFE.202..521D |url=https://www.researchgate.net/publication/224026482 |access-date=19 January 2025 }} and glacial moraines.{{cite journal |last1=Kabala |first1=Cesary |last2=Kubicz |first2=Justyna |year=2012 |title=Initial soil development and carbon accumulation on moraines of the rapidly retreating Werenskiold Glacier, SW Spitsbergen, Svalbard archipelago |journal=Geoderma |volume=175–176 |pages=9–20 |url=https://www.academia.edu/31221217 |doi=10.1016/j.geoderma.2012.01.025 |access-date=19 January 2025 |bibcode=2012Geode.175....9K }}

How soil formation proceeds is influenced by at least five classic factors that are intertwined in the evolution of a soil: parent material, climate, topography (relief), organisms, and time.{{cite book |last=Jenny |first=Hans |title=Factors of soil formation: a system of qunatitative pedology |year=1941 |publisher=McGraw-Hill |location=New York |url=https://netedu.xauat.edu.cn/sykc/hjx/content/ckzl/6/2.pdf |access-date=19 January 2025 |archive-url=https://web.archive.org/web/20170808104008/http://netedu.xauat.edu.cn/sykc/hjx/content/ckzl/6/2.pdf |archive-date=8 August 2017 |url-status=live }} When reordered to climate, relief, organisms, parent material, and time, they form the acronym CROPT.{{cite web |url=https://geo.libretexts.org/Bookshelves/Geography_(Physical)/The_Physical_Environment_(Ritter) |title=The physical environment: an introduction to physical geography |first=Michael E. |last=Ritter |date=4 July 2021 |access-date=19 January 2025 }}

{{main|Physical properties of soil}}

{{for|the academic discipline|Soil physics}}

The physical properties of soils, in order of decreasing importance for ecosystem services such as crop production, are texture, structure, bulk density, porosity, consistency, temperature, colour and resistivity.{{cite book |last1=Gardner |first1=Catriona M.K. |last2=Laryea |first2=Kofi Buna |last3=Unger |first3=Paul W. |date=1999 |title=Soil physical constraints to plant growth and crop production |edition=first |location=Rome, Italy |publisher=Food and Agriculture Organization of the United Nations |url=https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=8dc3f09583443adcb37d380bde37398f479386ba |archive-url=https://web.archive.org/web/20170808175354/http://www.plantstress.com/Files/Soil_Physical_Constraints.pdf |archive-date=8 August 2017 |url-status=live |access-date=26 January 2025 }} Soil texture is determined by the relative proportion of the three kinds of soil mineral particles, called soil separates: sand, silt, and clay. At the next larger scale, soil structures called peds or more commonly soil aggregates are created from the soil separates when iron oxides, carbonates, clay, silica and humus, coat particles and cause them to adhere into larger, relatively stable secondary structures.{{cite journal |last1=Six |first1=Johan |last2=Paustian |first2=Keith |last3=Elliott |first3=Edward T. |last4=Combrink |first4=Clay |journal=Soil Science Society of America Journal |volume=64 |issue=2 |title=Soil structure and organic matter. I. Distribution of aggregate-size classes and aggregate-associated carbon |url=https://www.researchgate.net/publication/280798601 |year=2000 |pages=681–689 |doi=10.2136/sssaj2000.642681x |access-date=26 January 2025 |bibcode=2000SSASJ..64..681S}} Soil bulk density, when determined at standardized moisture conditions, is an estimate of soil compaction.{{cite journal |last1=Håkansson |first1=Inge |last2=Lipiec |first2=Jerzy |journal=Soil and Tillage Research |volume=53 |issue=2 |title=A review of the usefulness of relative bulk density values in studies of soil structure and compaction |url=https://www.researchgate.net/publication/222541793 |year=2000 |pages=71–85 |doi=10.1016/S0167-1987(99)00095-1 |bibcode=2000STilR..53...71H |s2cid=30045538 |access-date=26 January 2025 |archive-date=16 May 2022 |archive-url=https://web.archive.org/web/20220516120555/http://directory.umm.ac.id/Data%20Elmu/jurnal/S/Soil%20%26%20Tillage%20Research/Vol53.Issue2.Jan2000/1452.pdf |url-status=live }} Soil porosity consists of the void part of the soil volume and is occupied by gases or water. Soil consistency is the ability of soil materials to stick together. Soil temperature and colour are self-defining. Resistivity refers to the resistance to conduction of electric currents and affects the rate of corrosion of metal and concrete structures which are buried in soil.{{cite journal |last=Schwerdtfeger |first=William J. |journal=Journal of Research of the National Bureau of Standards |volume=69C |issue=1 |title=Soil resistivity as related to underground corrosion and cathodic protection |year=1965 |pages=71–77 |doi=10.6028/jres.069c.012 |url=https://nvlpubs.nist.gov/nistpubs/jres/69C/jresv69Cn1p71_A1b.pdf |access-date=26 January 2025 }} These properties vary through the depth of a soil profile, i.e. through soil horizons. Most of these properties determine the aeration of the soil and the ability of water to infiltrate and to be held within the soil.{{cite book |last=Tamboli |first=Prabhakar Mahadeo |date=1961 |title=The influence of bulk density and aggregate size on soil moisture retention |location=Ames, Iowa |publisher=Iowa State University |url=https://dr.lib.iastate.edu/server/api/core/bitstreams/85621186-4b03-4140-ad1c-b18c3ab3b4a8/content |access-date=26 January 2025 }}

{{Main|Soil moisture}}

Soil water content can be measured as volume or weight. Soil moisture levels, in order of decreasing water content, are saturation, field capacity, wilting point, air dry, and oven dry. Field capacity describes a drained wet soil at the point water content reaches equilibrium with gravity. Irrigating soil above field capacity risks percolation losses. Wilting point describes the dry limit for growing plants. During growing season, soil moisture is unaffected by plant functional groups or species richness while it varies with species composition.{{Cite journal |last1=Spehn |first1=Eva M. |last2=Joshi |first2=Jasmin |last3=Schmid |first3=Bernhard |last4=Alphei |first4=Jörn |last5=Körner |first5=Christian |date=2000 |title=Plant diversity effects on soil heterotrophic activity in experimental grassland ecosystems |url=https://www.academia.edu/24032411 |journal=Plant and Soil |volume=224 |issue=2 |pages=217–230 |doi=10.1023/A:1004891807664|bibcode=2000PlSoi.224..217S |s2cid=25639544 |access-date=26 January 2025 }}

Available water capacity is the amount of water held in a soil profile available to plants. As water content drops, plants have to work against increasing forces of adhesion and sorptivity to withdraw water. Irrigation scheduling avoids moisture stress by replenishing depleted water before stress is induced.{{cite web |title=Water holding capacity |work=Oregon State University |date=24 June 2016 |url=https://forages.oregonstate.edu/ssis/soils/characteristics/water-holding-capacity |quote=Irrigators must have knowledge of the readily available moisture capacity so that water can be applied before plants have to expend excessive energy to extract moisture |access-date=26 January 2025 }}{{cite web |title=Basics of irrigation scheduling |work=University of Minnesota Extension |url=https://extension.umn.edu/irrigation/basics-irrigation-scheduling |quote=Only a portion of the available water holding capacity is easily used by the crop before crop water stress develop |access-date=26 January 2025 }}

Capillary action is responsible for moving groundwater from wet regions of the soil to dry areas. Subirrigation designs (e.g., wicking beds, sub-irrigated planters) rely on capillarity to supply water to plant roots. Capillary action can result in an evaporative concentration of salts, causing land degradation through salination.

Soil moisture measurement—measuring the water content of the soil, as can be expressed in terms of volume or weight—can be based on in situ probes (e.g., capacitance probes, neutron probes), or remote sensing methods. Soil moisture measurement is an important factor in determining changes in soil activity.

{{main|Soil gas}}

The atmosphere of soil, or soil gas, is very different from the atmosphere above. The consumption of oxygen by microbes and plant roots, and their release of carbon dioxide, decreases oxygen and increases carbon dioxide concentration. Atmospheric CO₂ concentration is 0.04%, but in the soil pore space it may range from 10 to 100 times that level, thus potentially contributing to the inhibition of root respiration.{{cite journal |last1=Qi |first1=Jingen |last2=Marshall |first2=John D. |last3=Mattson |first3=Kim G. |journal=New Phytologist |volume=128 |issue=3 |title=High soil carbon dioxide concentrations inhibit root respiration of Douglas fir |year=1994 |pages=435–442 |doi=10.1111/j.1469-8137.1994.tb02989.x |pmid=33874575 |doi-access=free |bibcode=1994NewPh.128..435Q }} Calcareous soils regulate CO₂ concentration by carbonate buffering, contrary to acid soils in which all CO₂ respired accumulates in the soil pore system.{{cite journal |last1=Karberg |first1=Noah J. |last2=Pregitzer |first2=Kurt S. |last3=King |first3=John S. |last4=Friend |first4=Aaron L. |last5=Wood |first5=James R. |journal=Oecologia |volume=142 |issue=2 |title=Soil carbon dioxide partial pressure and dissolved inorganic carbonate chemistry under elevated carbon dioxide and ozone |url=https://www.researchgate.net/publication/8337234 |year=2005 |pages=296–306 |doi=10.1007/s00442-004-1665-5 |pmid=15378342 |access-date=26 January 2025 |bibcode=2005Oecol.142..296K |s2cid=6161016 }} At extreme levels, CO₂ is toxic.{{cite journal |last1=Chang |first1=H.T. |last2=Loomis |first2=Walter E. |journal=Plant Physiology |volume=20 |issue=2 |title=Effect of carbon dioxide on absorption of water and nutrients by roots |year=1945 |pages=221–32 |doi=10.1104/pp.20.2.221 |pmid=16653979 |pmc=437214 |doi-access=free }} This suggests a possible negative feedback control of soil CO₂ concentration through its inhibitory effects on root and microbial respiration (also called soil respiration).{{cite journal |last1=McDowell |first1=Nate J. |last2=Marshall |first2=John D. |last3=Qi |first3=Jingen |last4=Mattson |first4=Kim |journal=Tree Physiology |volume=19 |issue=9 |title=Direct inhibition of maintenance respiration in western hemlock roots exposed to ambient soil carbon dioxide concentrations |year=1999 |pages=599–605 |doi=10.1093/treephys/19.9.599 |pmid=12651534 |doi-access=free }} In addition, the soil voids are saturated with water vapour, at least until the point of maximal hygroscopicity, beyond which a vapour-pressure deficit occurs in the soil pore space. Adequate porosity is necessary, not just to allow the penetration of water, but also to allow gases to diffuse in and out. Movement of gases is by diffusion from high concentrations to lower, the diffusion coefficient decreasing with soil compaction.{{cite journal |last1=Xu |first1=Xia |last2=Nieber |first2=John L. |last3=Gupta |first3=Satish C. |journal=Soil Science Society of America Journal |volume=56 |issue=6 |title=Compaction effect on the gas diffusion coefficient in soils |url=https://www.academia.edu/6547475 |year=1992 |pages=1743–1750 |doi=10.2136/sssaj1992.03615995005600060014x |access-date=26 January 2025 |bibcode=1992SSASJ..56.1743X }} Oxygen from above atmosphere diffuses in the soil where it is consumed and levels of carbon dioxide in excess of above atmosphere diffuse out with other gases (including greenhouse gases) as well as water.{{cite journal |last1=Smith |first1=Keith A. |last2=Ball |first2=Tom |last3=Conen |first3=Franz |last4=Dobbie |first4=Karen E. |last5=Massheder |first5=Jonathan |last6=Rey |first6=Ana |journal=European Journal of Soil Science |volume=54 |issue=4 |title=Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes |url=https://www.academia.edu/14433607 |year=2003 |pages=779–91 |doi=10.1046/j.1351-0754.2003.0567.x |bibcode=2003EuJSS..54..779S |s2cid=18442559 |access-date=26 January 2025 }} Soil texture and structure strongly affect soil porosity and gas diffusion. It is the total pore space (porosity) of soil, not the pore size, and the degree of pore interconnection (or conversely pore sealing), together with water content, air turbulence and temperature, that determine the rate of diffusion of gases into and out of soil.{{sfn|Russell|1957|pp=35–36}} Platy soil structure and soil compaction (low porosity) impede gas flow, and a deficiency of oxygen may encourage anaerobic bacteria to reduce (strip oxygen) from nitrate NO₃ to the gases N₂, N₂O, and NO, which are then lost to the atmosphere, thereby depleting the soil of nitrogen, a detrimental process called denitrification.{{cite journal |last1=Ruser |first1=Reiner |last2=Flessa |first2=Heiner |last3=Russow |first3=Rolf |last4=Schmidt |first4=G. |last5=Buegger |first5=Franz |last6=Munch |first6=J.C. |journal=Soil Biology and Biochemistry |volume=38 |issue=2 |title=Emission of N₂O, N₂ and CO₂ from soil fertilized with nitrate: effect of compaction, soil moisture and rewetting |url=https://downloads.regulations.gov/APHIS-2011-0023-0105/attachment_4.pdf |year=2006 |pages=263–74 |doi=10.1016/j.soilbio.2005.05.005 |access-date=26 January 2025 }} Aerated soil is also a net sink of methane (CH₄){{cite journal |last1=Hartmann |first1=Adrian A. |last2=Buchmann |first2=Nina |last3=Niklaus |first3=Pascal A. |journal=Plant and Soil |volume=342 |issue=1–2 |title=A study of soil methane sink regulation in two grasslands exposed to drought and N fertilization |year=2011 |pages=265–275 |doi=10.1007/s11104-010-0690-x |bibcode=2011PlSoi.342..265H |hdl=20.500.11850/34759 |s2cid=25691034 |url=https://www.research-collection.ethz.ch/bitstream/handle/20.500.11850/34759/2/11104_2010_Article_690.pdf |access-date=26 January 2025 }} but a net producer of methane (a strong heat-absorbing greenhouse gas) when soils are depleted of oxygen and subject to elevated temperatures.{{cite journal |last1=Moore |first1=Tim R. |last2=Dalva |first2=Moshe |journal=Journal of Soil Science |volume=44 |issue=4 |title=The influence of temperature and water table position on carbon dioxide and methane emissions from laboratory columns of peatland soils |url=https://www.researchgate.net/publication/229878721 |year=1993 |pages=651–64 |doi=10.1111/j.1365-2389.1993.tb02330.x |access-date=26 January 2025 }}

Soil atmosphere is also the seat of emissions of volatiles other than carbon and nitrogen oxides from various soil organisms, e.g. roots,{{cite journal |last1=Hiltpold |first1=Ivan |last2=Toepfer |first2=Stefan |last3=Kuhlmann |first3=Ulrich |last4=Turlings |first4=Ted C.J. |journal=Chemoecology |volume=20 |issue=2 |title=How maize root volatiles affect the efficacy of entomopathogenic nematodes in controlling the western corn rootworm? |url=https://www.researchgate.net/publication/215470509 |year=2010 |pages=155–62 |doi=10.1007/s00049-009-0034-6 |bibcode=2010Chmec..20..155H |s2cid=30214059 |access-date=2 February 2025 }} bacteria,{{cite journal |last1=Ryu |first1=Choong-Min |last2=Farag |first2=Mohamed A. |last3=Hu |first3=Chia-Hui |last4=Reddy |first4=Munagala S. |last5= Wei |first5= Han-Xun |last6= Paré |first6=Paul W. |last7= Kloepper |first7= Joseph W. |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=100 |issue=8 |title=Bacterial volatiles promote growth in Arabidopsis |year=2003 |pages=4927–32 |doi=10.1073/pnas.0730845100 |pmid=12684534 |pmc=153657 |bibcode=2003PNAS..100.4927R |doi-access=free }} fungi,{{cite journal |last1=Hung |first1=Richard |last2=Lee |first2=Samantha |last3=Bennett |first3=Joan W. |journal=Applied Microbiology and Biotechnology |volume=99 |issue=8 |title=Fungal volatile organic compounds and their role in ecosystems |url=https://www.researchgate.net/publication/273638498 |year=2015 |pages=3395–405 |doi=10.1007/s00253-015-6494-4 |pmid=25773975 |s2cid=14509047 |access-date=2 February 2025 }} animals.{{cite journal |last1=Purrington |first1=Foster Forbes |last2=Kendall |first2=Paricia A. |last3=Bater |first3=John E. |last4=Stinner |first4=Benjamin R. |journal=Great Lakes Entomologist |volume=24 |issue=2 |title=Alarm pheromone in a gregarious poduromorph collembolan (Collembola: Hypogastruridae) |url=https://scholar.valpo.edu/cgi/viewcontent.cgi?article=1732&context=tgle |year=1991 |pages=75–8 |access-date=2 February 2025 }} These volatiles are used as chemical cues, making soil atmosphere the seat of interaction networks{{cite journal |last1=Badri |first1=Dayakar V. |last2=Weir |first2=Tiffany L. |last3=Van der Lelie |first3=Daniel |last4=Vivanco |first4=Jorge M |journal=Current Opinion in Biotechnology |volume=20 |issue=6 |title=Rhizosphere chemical dialogues: plant–microbe interactions |url=https://www.bicga.org.uk/docs/Rhizosphere_chemical_dialogues_plant.pdf |doi=10.1016/j.copbio.2009.09.014 |pmid=19875278 |year=2009 |pages=642–50 |access-date=2 February 2025 |archive-date=21 September 2022 |archive-url=https://web.archive.org/web/20220921224421/http://www.bicga.org.uk/docs/Rhizosphere_chemical_dialogues_plant.pdf |url-status=live }}{{cite journal |last1=Salmon |first1=Sandrine |last2=Ponge |first2=Jean-François |journal=Soil Biology and Biochemistry |volume=33 |issue=14 |title=Earthworm excreta attract soil springtails: laboratory experiments on Heteromurus nitidus (Collembola: Entomobryidae) |url=https://www.academia.edu/20508985 |doi=10.1016/S0038-0717(01)00129-8 |year=2001 |pages=1959–69 |bibcode=2001SBiBi..33.1959S |s2cid=26647480 |access-date=2 February 2025 }} playing a decisive role in the stability, dynamics and evolution of soil ecosystems.{{cite journal |last1=Lambers |first1=Hans |last2=Mougel |first2=Christophe |last3=Jaillard |first3=Benoît |last4=Hinsinger |first4=Philipe |journal=Plant and Soil |volume=321 |issue=1–2 |title=Plant-microbe-soil interactions in the rhizosphere: an evolutionary perspective |url=https://www.academia.edu/25517742 |doi=10.1007/s11104-009-0042-x |year=2009 |pages=83–115 |bibcode=2009PlSoi.321...83L |s2cid=6840457 |access-date=2 February 2025 }} Biogenic soil volatile organic compounds are exchanged with the aboveground atmosphere, in which they are just 1–2 orders of magnitude lower than those from aboveground vegetation.{{cite journal |last1=Peñuelas |first1=Josep |last2=Asensio |first2=Dolores |last3=Tholl |first3=Dorothea |last4=Wenke |first4=Katrin |last5=Rosenkranz |first5=Maaria |last6=Piechulla |first6=Birgit |last7=Schnitzler |first7=Jörg-Petter |journal=Plant, Cell and Environment |volume=37 |issue=8 |title=Biogenic volatile emissions from the soil |year=2014 |pages=1866–91 |doi=10.1111/pce.12340 |pmid=24689847 |doi-access=free |bibcode=2014PCEnv..37.1866P }}

Humans can get some idea of the soil atmosphere through the well-known 'after-the-rain' scent, when infiltering rainwater flushes out the whole soil atmosphere after a drought period, or when soil is excavated,{{cite journal |last1=Buzuleciu |first1=Samuel A. |last2=Crane |first2=Derek P. |last3=Parker |first3=Scott L. |journal=Herpetological Conservation and Biology |volume=11 |issue=3 |title=Scent of disinterred soil as an olfactory cue used by raccoons to locate nests of diamond-backed terrapins (Malaclemys terrapin) |url=https://www.herpconbio.org/Volume_11/Issue_3/Buzuleciu_etal_2016.pdf |year=2016 |pages=539–51 |access-date=2 February 2025 }} a bulk property attributed in a reductionist manner to particular biochemical compounds such as petrichor or geosmin.

{{main|Soil matrix}}

Soil particles can be classified by their chemical composition (mineralogy) as well as their size. The particle size distribution of a soil, its texture, determines many of the properties of that soil, in particular hydraulic conductivity and water potential,{{cite journal |last1=Saxton |first1=Keith E. |last2=Rawls |first2=Walter J. |journal=Soil Science Society of America Journal |volume=70 |issue=5 |title=Soil water characteristic estimates by texture and organic matter for hydrologic solutions |url=https://www.atmos.illinois.edu/~sshu3/model/saxton2006.pdf |archive-url=https://web.archive.org/web/20180902183902/https://pdfs.semanticscholar.org/5e63/c886c4f68af5e5c242c006d2d882f0a65bfe.pdf |url-status=live |archive-date=2 September 2018 |year=2006 |pages=1569–78 |doi=10.2136/sssaj2005.0117 |access-date=2 February 2025 |bibcode=2006SSASJ..70.1569S |s2cid=16826314 }} but the mineralogy of those particles can strongly modify those properties. The mineralogy of the finest soil particles, clay, is especially important.{{cite web |last=College of Tropical Agriculture and Human Resources |title=Soil mineralogy |url=https://www.ctahr.hawaii.edu/mauisoil/a_factor_mineralogy.aspx |publisher=University of Hawaiʻi at Mānoa |access-date=2 February 2025 }}

Large numbers of microbes, animals, plants and fungi are living in soil.{{cite book |last1=Dessaux |first1=Yves |last2=Chapelle |first2=Émilie |last3=Faure |first3=Denis |date=20 September 2010 |chapter=Quorum sensing and quorum quenching in soil ecosystems |title=Biocommunication in soil microorganisms |edition=1st |editor-last=Witzany |editor-first=Günther |publisher=Springer Nature |location=Berlin, Germany |pages=339–67 |isbn=978-3-642-14512-4 |chapter-url=https://www.researchgate.net/publication/225865218 |doi=10.1007/978-3-642-14512-4_13 |access-date=13 April 2025 }} However, biodiversity in soil is much harder to study as most of this life is invisible, hence estimates about soil biodiversity have been unsatisfactory. A recent study suggested that soil is likely home to 59 ± 15% of the species on Earth. Enchytraeidae (potworms) have the greatest percentage of their species living in soil (98.6%), followed by fungi (90%), plants (85.5%), and termites (Isoptera) (84.2%). Many other groups of animals have substantial fractions of species living in soil, e.g. about 30% of insects, and close to 50% of arachnids.{{cite journal |last1=Anthony |first1=Mark A. |last2=Bender |first2=S. Franz |last3=Van der Heijden |first3=Marcel G. A. |date=2023-08-15 |title=Enumerating soil biodiversity |journal=Proceedings of the National Academy of Sciences of the United States of America |language=en |volume=120 |issue=33 |pages=e2304663120 |doi=10.1073/pnas.2304663120 |pmid=37549278 |pmc=10437432 |bibcode=2023PNAS..12004663A |issn=0027-8424|doi-access=free }} While most vertebrates live above ground (ignoring aquatic species), many species are fossorial, that is, they live in soil, such as most blind snakes.

{{for|the academic discipline|Soil chemistry}}

The chemistry of a soil determines its ability to supply available plant nutrients and affects its physical properties and the health of its living population. In addition, a soil's chemistry also determines its corrosivity, stability, and ability to absorb pollutants and to filter water. It is the surface chemistry of mineral and organic colloids that determines soil's chemical properties.{{cite book |last=Sposito |first=Garrison |date=1984 |title=The surface chemistry of soils |publisher=Oxford University Press |location=New York |url=https://epdf.pub/the-surface-chemistry-of-soils.html |access-date=9 February 2025 }} A colloid is a small, insoluble particle ranging in size from 1 nanometer to 1 micrometer, thus small enough to remain suspended by Brownian motion in a fluid medium without settling.{{cite web |last=Wynot |first=Christopher |title=Theory of diffusion in colloidal suspensions |url=http://www.owlnet.rice.edu/~ceng402/proj02/cwynot/402project.htm |access-date=9 February 2025 }} Most soils contain organic colloidal particles called humus as well as the inorganic colloidal particles of clays. The very high specific surface area of colloids and their net electrical charges give soil its ability to hold and release ions. Negatively charged sites on colloids attract and release cations in what is referred to as cation exchange. Cation-exchange capacity is the amount of exchangeable cations per unit weight of dry soil and is expressed in terms of milliequivalents of positively charged ions per 100 grams of soil (or centimoles of positive charge per kilogram of soil; cmol_c/kg). Similarly, positively charged sites on colloids can attract and release anions in the soil, giving the soil anion exchange capacity.

{{Further|Cation-exchange capacity}}

The cation exchange, that takes place between colloids and soil water, buffers (moderates) soil pH, alters soil structure, and purifies percolating water by adsorbing cations of all types, both useful and harmful.

The negative or positive charges on colloid particles make them able to hold cations or anions, respectively, to their surfaces. The charges result from four sources.{{sfn|Donahue|Miller|Shickluna|1977|p=103–106}}

1. Isomorphous substitution occurs in clay during its formation, when lower-valence cations substitute for higher-valence cations in the crystal structure.{{cite journal |last1=Sposito |first1= Garrison |last2=Skipper |first2=Neal T. |last3=Sutton |first3=Rebecca |last4=Park |first4=Sung-Ho |last5=Soper |first5=Alan K. |last6=Greathouse |first6=Jeffery A. |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=96 |issue=7 |title=Surface geochemistry of the clay minerals |year=1999 |pages=3358–64 |doi=10.1073/pnas.96.7.3358 |pmid=10097044 |bibcode=1999PNAS...96.3358S |pmc=34275 |doi-access=free }} Substitutions in the outermost layers are more effective than for the innermost layers, as the electric charge strength drops off as the square of the distance. The net result is oxygen atoms with net negative charge and the ability to attract cations.
2. Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete.{{cite journal |last1=Bickmore |first1=Barry R. |last2=Rosso |first2=Kevin M. |last3=Nagy |first3=Kathryn L. |last4=Cygan |first4=Randall T. |last5=Tadanier |first5=Christopher J. |year=2003 |title=Ab initio determination of edge surface structures for dioctahedral 2:1 phyllosilicates: implications for acid-base reactivity |journal=Clays and Clay Minerals |volume=51 |issue=4 |pages=359–71 |url=https://randallcygan.com/wp-content/uploads/2017/06/Bickmore2003CCM.pdf |doi=10.1346/CCMN.2003.0510401 |access-date=9 February 2025 |bibcode=2003CCM....51..359B |s2cid=97428106 }}
3. Hydroxyls may substitute for oxygens of the silica layers, a process called hydroxylation. When the hydrogens of the clay hydroxyls are ionised into solution, they leave the oxygen with a negative charge (anionic clays).{{cite journal |last1=Rajamathi |first1=Michael |last2=Thomas |first2=Grace S. |last3=Kamath |first3=P. Vishnu |year=2001 |title=The many ways of making anionic clays |journal=Journal of Chemical Sciences |volume=113 |issue=5–6 |pages=671–80 |doi=10.1007/BF02708799 |s2cid=97507578 |url=https://www.academia.edu/56207482 |access-date=9 February 2025 }}
4. Hydrogens of humus hydroxyl groups may also be ionised into solution, leaving, similarly to clay, an oxygen with a negative charge.{{cite journal |last1=Moayedi |first1=Hossein |last2=Kazemian |first2=Sina |year= 2012 |title=Zeta potentials of suspended humus in multivalent cationic saline solution and its effect on electro-osomosis behavior |journal=Journal of Dispersion Science and Technology |volume=34 |issue=2 |pages=283–94 |url=https://www.academia.edu/10587240 |doi=10.1080/01932691.2011.646601 |s2cid= 94333872 |access-date=9 February 2025 }}

Cations held to the negatively charged colloids resist being washed downward by water and are at first out of reach of plant roots, thereby preserving the soil fertility in areas of moderate rainfall and low temperatures.{{cite web |last=Pettit |first=Robert E. |title=Organic matter, humus, humate, humic acid, fulvic acid and humin: their importance in soil fertility and plant health |url=https://humates.com/wp-content/uploads/2020/04/ORGANICMATTERPettit.pdf |access-date=16 February 2025 }}{{cite journal |last1=Diamond |first1=Sidney |last2=Kinter |first2=Earl B. |year=1965 |title=Mechanisms of soil-lime stabilization: an interpretive review |journal=Highway Research Record |volume=92 |pages=83–102 |url=https://onlinepubs.trb.org/onlinepubs/hrr/1965/92/92-006.pdf |access-date=16 February 2025 }}

There is a hierarchy in the process of cation exchange on colloids, as cations differ in the strength of adsorption by the colloid and hence their ability to replace one another (ion exchange). If present in equal amounts in the soil water solution:

Al³⁺ replaces H⁺ replaces Ca²⁺ replaces Mg²⁺ replaces K⁺ same as {{chem|NH|4|+}} replaces Na⁺{{cite journal |last=Woodruff |first=Clarence M. |year=1955 |title=The energies of replacement of calcium by potassium in soils |journal=Soil Science Society of America Journal |volume=19 |issue=2 |pages=167–71 |doi=10.2136/sssaj1955.03615995001900020014x |url=https://www.ipipotash.org/uploads/pdf/review/30_1956_1.pdf |bibcode=1955SSASJ..19..167W |access-date=16 February 2025 }}

If one cation is added in large amounts, it may replace the others by the sheer force of its numbers. This is called law of mass action. This is largely what occurs with the addition of cationic fertilisers (potash, lime).{{cite journal |last=Fronæus |first=Sture |year=1953 |title=On the application of the mass action law to cation exchange equilibria |journal=Acta Chemica Scandinavica |volume=7 |pages=469–80 |doi=10.3891/acta.chem.scand.07-0469 |doi-access=free }}

As the soil solution becomes more acidic (low pH, meaning an abundance of H⁺), the other cations more weakly bound to colloids are pushed into solution as hydrogen ions occupy exchange sites (protonation). A low pH may cause the hydrogen of hydroxyl groups to be pulled into solution, leaving charged sites on the colloid available to be occupied by other cations. This ionisation of hydroxy groups on the surface of soil colloids creates what is described as pH-dependent surface charges.{{cite journal |last1=Bolland |first1=Mike D. A. |last2=Posner |first2=Alan M. |last3=Quirk |first3=James P. |year=1980 |title=pH-independent and pH-dependent surface charges on kaolinite |journal=Clays and Clay Minerals |volume=28 |issue=6 |pages=412–18 |doi=10.1346/CCMN.1980.0280602 |bibcode=1980CCM....28..412B |s2cid=12462516 |doi-access=free }} Unlike permanent charges developed by isomorphous substitution, pH-dependent charges are variable and increase with increasing pH.{{cite web |last=Chakraborty |first=Somsubhra |url=http://elearn.psgcas.ac.in/nptel/courses/video/126105016/lec24.pdf |date=2 February 2019 |title=Cation Exchange Capacity (CEC) |access-date=16 February 2025 }} Freed cations can be made available to plants but are also prone to be leached from the soil, possibly making the soil less fertile.{{cite journal |last1=Silber |first1=Avner |last2=Levkovitch |first2=Irit |last3= Graber |first3=Ellen R. |year=2010 |title=pH-dependent mineral release and surface properties of cornstraw biochar: agronomic implications |journal=Environmental Science and Technology |volume=44 |issue=24 |pages=9318–23 |url=https://www.academia.edu/24532141 |doi=10.1021/es101283d |pmid=21090742 |access-date=16 February 2025 |bibcode=2010EnST...44.9318S }} Plants are able to excrete H⁺ into the soil through the synthesis of organic acids and by that means, change the pH of the soil near the root and push cations off the colloids, thus making those available to the plant.{{cite journal |last1=Dakora |first1=Felix D. |last2=Phillips |first2=Donald D. |year=2002 |title=Root exudates as mediators of mineral acquisition in low-nutrient environments |journal=Plant and Soil |volume=245 |issue=1 |pages=35–47 |url=https://www.researchgate.net/publication/225265745 |doi=10.1023/A:1020809400075 |bibcode=2002PlSoi.245...35D |s2cid=3330737 |access-date=16 February 2025 |archive-url=https://web.archive.org/web/20190819123707/http://www.plantstress.com/articles/min_deficiency_i/root_exudates.pdf |archive-date=19 August 2019 |url-status=live }}

Cation exchange capacity is the soil's ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution.{{cite journal |last=Brown |first=John C. |year=1978 |title=Mechanism of iron uptake by plants |journal=Plant, Cell and Environment |volume=1 |issue=4 |pages=249–57 |doi=10.1111/j.1365-3040.1978.tb02037.x |bibcode=1978PCEnv...1..249B |url=https://fr.1lib.sk/book/41304841/1381d1 |access-date=16 February 2025 }} CEC is the amount of exchangeable hydrogen cations (H⁺) that will combine with 100 grams dry weight of soil and whose measure is one milliequivalent per 100 grams of soil (1 meq/100 g). Hydrogen ions have a single charge and one-thousandth of a gram of hydrogen ions per 100 grams dry soil gives a measure of one milliequivalent of hydrogen ion. Calcium, with an atomic weight 40 times that of hydrogen and with a valence of two, converts to {{nowrap|(40 ÷ 2) × 1 milliequivalent}} = 20 milliequivalents of hydrogen ion per 100 grams of dry soil or 20 meq/100 g.{{sfn|Donahue|Miller|Shickluna|1977|p=114}} The modern measure of CEC is expressed as centimoles of positive charge per kilogram (cmol/kg) of oven-dry soil.

Most of the soil's CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates (such as tropical rainforests), due to leaching and decomposition, respectively, explains the apparent sterility of tropical soils.{{cite journal |last1=Singh |first1=Jamuna Sharan |last2=Raghubanshi |first2=Akhilesh Singh |last3=Singh |first3=Raj S. |last4=Srivastava |first4=S. C. |year=1989 |title=Microbial biomass acts as a source of plant nutrient in dry tropical forest and savanna |journal=Nature |volume=338 |issue=6215 |pages=499–500 |url=https://www.researchgate.net/publication/236941524 |doi=10.1038/338499a0 |access-date=16 February 2025 |bibcode=1989Natur.338..499S |s2cid=4301023 }} Live plant roots also have some CEC, linked to their specific surface area.{{cite journal |last1=Szatanik-Kloc |first1=Alicja |last2=Szerement |first2=Justyna |last3=Józefaciuk |first3=Grzegorz |year=2017 |title=The role of cell walls and pectins in cation exchange and surface area of plant roots |journal=Journal of Plant Physiology |volume=215 |pages=85–90 |url=https://daneshyari.com/article/preview/5517999.pdf |doi=10.1016/j.jplph.2017.05.017 |pmid=28600926 |bibcode=2017JPPhy.215...85S |access-date=16 February 2025 }}

{| class="wikitable" style="border-spacing: 5px; margin:auto;"

|+ Cation exchange capacity for soils; soil textures; soil colloids{{sfn|Donahue|Miller|Shickluna|1977|pp=115–116}}

|-

! scope="col" style="width:200px;"| Soil

! scope="col" style="width:100px;"| State

! scope="col" style="width:100px;"| CEC meq/100 g

|-

| Charlotte fine sand ||Florida|| 1.0

|-

| Ruston fine sandy loam ||Texas|| 1.9

|-

| Glouchester loam ||New Jersey || 11.9

|-

| Grundy silt loam || Illinois || 26.3

|-

| Gleason clay loam || California || 31.6

|-

| Susquehanna clay loam || Alabama || 34.3

|-

| Davie mucky fine sand || Florida || 100.8

|-

| Sands || {{n/a}} || 1–5

|-

| Fine sandy loams || {{n/a}} || 5–10

|-

| Loams and silt loams || {{n/a}} || 5–15

|-

| Clay loams || {{n/a}} || 15–30

|-

| Clays || {{n/a}} || over 30

|-

| Sesquioxides || {{n/a}} || 0–3

|-

| Kaolinite || {{n/a}} || 3–15

|-

| Illite || {{n/a}} || 25–40

|-

| Montmorillonite || {{n/a}} || 60–100

|-

| Vermiculite (similar to illite) || {{n/a}} || 80–150

|-

| Humus || {{n/a}} || 100–300

|}

Anion exchange capacity is the soil's ability to remove anions (such as nitrate, phosphate) from the soil water solution and sequester those for later exchange as the plant roots release carbonate anions to the soil water solution.{{cite journal |last= Hinsinger |first=Philippe |year=2001 |title=Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: a review |journal=Plant and Soil |volume=237 |issue=2 |pages=173–95 |doi=10.1023/A:1013351617532 |bibcode=2001PlSoi.237..173H |s2cid=8562338 |url=https://www.researchgate.net/publication/225852665 |access-date=23 February 2025 }} Those colloids which have low CEC tend to have some AEC. Amorphous and sesquioxide clays have the highest AEC,{{cite report |last1=Gu |first1=Baohua |last2=Schulz |first2=Robert K. |title=Anion retention in soil: possible application to reduce migration of buried technetium and iodine, a review |year=1991 |doi=10.2172/5980032 |s2cid=91359494 |url=https://www.osti.gov/servlets/purl/5980032 |access-date=23 February 2025 }} followed by the iron oxides.{{cite journal |last1=Lawrinenko |first1=Michael |last2=Jing |first2=Dapeng |last3=Banik |first3=Chumki |last4=Laird |first4=David A. |year=2017 |title=Aluminum and iron biomass pretreatment impacts on biochar anion exchange capacity |journal=Carbon |volume=118 |pages=422–30 |doi=10.1016/j.carbon.2017.03.056 |bibcode=2017Carbo.118..422L |url=https://www.academia.edu/90757446 |access-date=23 February 2025 }} Levels of AEC are much lower than for CEC, because of the generally higher rate of positively (versus negatively) charged surfaces on soil colloids, to the exception of variable-charge soils.{{cite journal |last1=Sollins |first1=Phillip |last2=Robertson |first2=G. Philip |last3=Uehara |first3=Goro |year=1988 |title=Nutrient mobility in variable- and permanent-charge soils |journal=Biogeochemistry |volume=6 |issue=3 |pages=181–99 |url=https://lter.kbs.msu.edu/docs/robertson/Sollins_et_al._1988_Biogeochemistry.pdf |doi=10.1007/BF02182995 |bibcode=1988Biogc...6..181S |s2cid=4505438 |access-date=23 February 2025 }} Phosphates tend to be held at anion exchange sites.{{cite journal |last=Sanders |first=W. M. H. |year=1964 |title=Extraction of soil phosphate by anion-exchange membrane |journal=New Zealand Journal of Agricultural Research |volume=7 |issue=3 |pages=427–31 |doi=10.1080/00288233.1964.10416423 |bibcode=1964NZJAR...7..427S |doi-access=free }}

Iron and aluminum hydroxide clays are able to exchange their hydroxide anions (OH⁻) for other anions. The order reflecting the strength of anion adhesion is as follows:

:{{chem|H|2|PO|4|−}} replaces {{chem|SO|4|2−}} replaces {{chem|NO|3|−}} replaces Cl⁻

The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100 g dry soil.{{sfn|Donahue|Miller|Shickluna|1977|pp=115–116}} As pH rises, there are relatively more hydroxyls, which will displace anions from the colloids and force them into solution and out of storage; hence AEC decreases with increasing pH (alkalinity).{{cite journal |last1=Lawrinenko |first1=Mike |last2=Laird |first2=David A. |year=2015 |title=Anion exchange capacity of biochar |journal=Green Chemistry |volume=17 |issue=9 |pages=4628–36 |doi=10.1039/C5GC00828J |s2cid=52972476 |url=https://pubs.rsc.org/en/content/getauthorversionpdf/c5gc00828j |access-date=23 February 2025 }}

{{Main|Soil pH|Soil pH#Effect of soil pH on plant growth}}

Soil reactivity is expressed in terms of pH and is a measure of the acidity or alkalinity of the soil. More precisely, it is a measure of hydronium concentration in an aqueous solution and ranges in values from 0 to 14 (acidic to basic) but practically speaking for soils, pH ranges from 3.5 to 9.5, as pH values beyond those extremes are toxic to life forms.{{cite web |last=Robertson |first=Bryan |title=pH requirements of freshwater aquatic life |url=https://www.waterboards.ca.gov/waterrights/water_issues/programs/bay_delta/deltaflow/docs/exhibits/bigbreak/dscbb_exh5.pdf |access-date=23 February 2025 |archive-date=8 May 2021 |archive-url=https://web.archive.org/web/20210508070517/https://www.waterboards.ca.gov/centralvalley/water_issues/basin_plans/ph_turbidity/ph_turbidity_04phreq.pdf |url-status=live }}

At 25 °C an aqueous solution that has a pH of 3.5 has 10^−3.5 moles H₃O⁺ (hydronium ions) per litre of solution (and also 10^−10.5 moles per litre OH⁻). A pH of 7, defined as neutral, has 10⁻⁷ moles of hydronium ions per litre of solution and also 10⁻⁷ moles of OH⁻ per litre; since the two concentrations are equal, they are said to neutralise each other. A pH of 9.5 has 10^−9.5 moles hydronium ions per litre of solution (and also 10^−2.5 moles per litre OH⁻). A pH of 3.5 has one million times more hydronium ions per litre than a solution with pH of 9.5 ({{nowrap|9.5 − 3.5 {{=}} 6}} or 10⁶) and is more acidic.{{cite book |editor-last=Chang |editor-first=Raymond |title=Chemistry |date=2010 |edition=12th |url=https://www.academia.edu/44394574 |publisher=McGraw-Hill |location=New York, New York |isbn=9780078021510 |page=666 |access-date=23 February 2025 }}

The effect of pH on a soil is to remove from the soil or to make available certain ions. Soils with high acidity tend to have toxic amounts of aluminium and manganese.{{cite journal |last1=Rajamathi |first1=Michael |last2=Thomas |first2=Grace S. |last3=Kamath |first3=P. Vishnu |date=October 2001 |title=The many ways of making anionic clays |journal=Journal of Chemical Sciences |volume=113 |issue=5–6 |pages=671–80 |doi=10.1007/BF02708799 |s2cid=97507578 |url=https://www.researchgate.net/publication/226095576 |access-date=23 February 2025 }} As a result of a trade-off between toxicity and requirement most nutrients are better available to plants at moderate pH,{{cite book |last1=Läuchli |first1=André |last2=Grattan |first2=Steve R. |date=2012 |chapter=Soil pH extremes |title=Plant stress physiology |edition=1st |editor-first=Sergey |editor-last=Shabala |publisher=CAB International |location=Wallingford, United Kingdom |pages=194–209 |isbn=978-1845939953 |chapter-url=https://www.researchgate.net/publication/269112359 |doi=10.1079/9781845939953.0194 |access-date=23 February 2025 }} although most minerals are more soluble in acid soils. Soil organisms are hindered by high acidity, and most agricultural crops do best with mineral soils of pH 6.5 and organic soils of pH 5.5.{{sfn|Donahue|Miller|Shickluna|1977|pp=116–117}} Given that at low pH toxic metals (e.g. cadmium, zinc, lead) are positively charged as cations and organic pollutants are in non-ionic form, thus both made more available to organisms,{{cite journal |last1=Calmano |first1=Wolfgang |last2=Hong |first2=Jihua |last3=Förstner |first3=Ulrich |year=1993 |title=Binding and mobilization of heavy metals in contaminated sediments affected by pH and redox potential |journal=Water Science and Technology |volume=28 |issue=8–9 |pages=223–35 |url=https://www.researchgate.net/publication/234056281 |doi=10.2166/wst.1993.0622 |bibcode=1993WSTec..28..223C |access-date=23 February 2025 }}{{cite journal |last1=Ren |first1=Xiaoya |last2=Zeng |first2=Guangming |last3=Tang |first3=Lin |last4=Wang |first4=Jingjing |last5=Wan |first5=Jia |last6=Liu |first6=Yani |last7=Yu |first7=Jiangfang |last8=Yi |first8=Huan |last9=Ye |first9=Shujing |last10=Deng |first10=Rui |year=2018 |title=Sorption, transport and biodegradation: an insight into bioavailability of persistent organic pollutants in soil |journal=Science of the Total Environment |volume=610–611 |pages=1154–1163 |url=http://ee.hnu.edu.cn/__local/E/E3/44/F76DCA19501AE153573A22D4C29_17709BE2_110161.pdf |doi=10.1016/j.scitotenv.2017.08.089 |pmid=28847136 |access-date=23 February 2025 |bibcode=2018ScTEn.610.1154R }} it has been suggested that plants, animals and microbes commonly living in acid soils are pre-adapted to every kind of pollution, whether of natural or human origin.{{cite journal |last=Ponge |first=Jean-François |year=2003 |title=Humus forms in terrestrial ecosystems: a framework to biodiversity |journal=Soil Biology and Biochemistry |volume=35 |issue=7 |pages=935–45 |url=https://www.academia.edu/20508983 |doi=10.1016/S0038-0717(03)00149-4 |bibcode=2003SBiBi..35..935P |access-date=23 February 2025 |citeseerx=10.1.1.467.4937 |s2cid=44160220 }}

In high rainfall areas, soils tend to acidify as the basic cations are forced off the soil colloids by the mass action of hydronium ions from usual or unusual rain acidity against those attached to the colloids. High rainfall rates can then wash the nutrients out, leaving the soil inhabited only by those organisms which are particularly efficient to uptake nutrients in very acid conditions, like in tropical rainforests.{{cite journal |last=Fujii |first=Kazumichi |year=2003 |title=Soil acidification and adaptations of plants and microorganisms in Bornean tropical forests |journal=Ecological Research |volume=29 |issue=3 |pages=371–81 |doi=10.1007/s11284-014-1144-3 |doi-access=free }} Once the colloids are saturated with H₃O⁺, the addition of any more hydronium ions or aluminum hydroxyl cations drives the pH even lower (more acidic) as the soil has been left with no buffering capacity.{{cite journal |last1=Kauppi |first1=Pekka |last2=Kämäri |first2=Juha |last3=Posch |first3=Maximilian |last4=Kauppi |first4=Lea |year=1986 |title=Acidification of forest soils: model development and application for analyzing impacts of acidic deposition in Europe |journal=Ecological Modelling |volume=33 |issue=2–4 |pages=231–53 |url=https://pure.iiasa.ac.at/id/eprint/2766/1/RR-87-05.pdf |doi=10.1016/0304-3800(86)90042-6 |bibcode=1986EcMod..33..231K |access-date=2 March 2025 }} In areas of extreme rainfall and high temperatures, the clay and humus may be washed out, further reducing the buffering capacity of the soil.{{cite journal |last=Andriesse |first=Jacobus Pieter |year=1969 |title=A study of the environment and characteristics of tropical podzols in Sarawak (East-Malaysia) |journal=Geoderma |volume=2 |issue=3 |pages=201–27 |url=https://fr.1lib.sk/book/48380141/a3a1fd |doi=10.1016/0016-7061(69)90038-X |access-date=2 March 2025 |bibcode=1969Geode...2..201A }} In low rainfall areas, unleached calcium pushes pH to 8.5 and with the addition of exchangeable sodium, soils may reach pH 10.{{cite journal |last=Rengasamy |first=Pichu |year=2006 |title=World salinization with emphasis on Australia |journal=Journal of Experimental Botany |volume=57 |issue=5 |pages=1017–23 |doi=10.1093/jxb/erj108 |pmid=16510516 |url=https://www.researchgate.net/publication/7266400 |access-date=2 March 2025 }} Beyond a pH of 9, plant growth is reduced.{{cite journal |last1=Arnon |first1=Daniel I. |last2=Johnson |first2=Clarence M. |year=1942 |title=Influence of hydrogen ion concentration on the growth of higher plants under controlled conditions |journal=Plant Physiology |volume=17 |issue=4 |pages=525–39 |doi=10.1104/pp.17.4.525 |pmid=16653803 |pmc=438054 |url=https://fr.1lib.sk/book/80127175/fb0849 |access-date=2 March 2025 }} High pH results in low micro-nutrient mobility, but water-soluble chelates of those nutrients can correct the deficit.{{cite journal |last1=Chaney |first1=Rufus L. |last2=Brown |first2=John C. |last3=Tiffin |first3=Lee O. |year=1972 |title=Obligatory reduction of ferric chelates in iron uptake by soybeans |journal=Plant Physiology |volume=50 |issue=2 |pages=208–13 |doi=10.1104/pp.50.2.208 |pmid=16658143 |pmc=366111 |url=https://www.researchgate.net/publication/7123454 |access-date=2 March 2025 }} Sodium can be reduced by the addition of gypsum (calcium sulphate) as calcium adheres to clay more tightly than does sodium causing sodium to be pushed into the soil water solution where it can be washed out by an abundance of water.{{sfn|Donahue|Miller|Shickluna|1977|pp=116–119}}{{cite journal |last1=Ahmad |first1=Sagheer |last2=Ghafoor |first2=Abdul |last3=Qadir |first3=Manzoor |last4=Aziz |first4=M. Abbas |year=2006 |title=Amelioration of a calcareous saline-sodic soil by gypsum application and different crop rotations |journal=International Journal of Agriculture and Biology |volume=8 |issue=2 |pages=142–46 |url=https://www.researchgate.net/publication/228966353 |access-date=2 March 2025 }}

There are acid-forming cations (e.g. hydronium, aluminium, iron) and there are base-forming cations (e.g. calcium, magnesium, sodium). The fraction of the negatively-charged soil colloid exchange sites (CEC) that are occupied by base-forming cations is called base saturation. If a soil has a CEC of 20 meq and 5 meq are aluminium and hydronium cations (acid-forming), the remainder of positions on the colloids ({{nowrap|1=20 − 5 = 15 meq}}) are assumed occupied by base-forming cations, so that the base saturation is {{nowrap|1=15 ÷ 20 × 100% = 75%}} (the compliment 25% is assumed acid-forming cations). Base saturation is almost in direct proportion to pH (it increases with increasing pH).{{cite journal |last1=McFee |first1=William W. |last2=Kelly |first2=J. Michael |last3=Beck |first3=Robert H. |year=1977 |title=Acid precipitation effects on soil pH and base saturation of exchange sites |journal=Water, Air, and Soil Pollution |volume=7 |issue=3 |pages=401–08 |doi=10.1007/BF00284134 |bibcode=1977WASP....7..401M |url=https://www.researchgate.net/publication/226736129 |access-date=2 March 2025 }} It is of use in calculating the amount of lime needed to neutralise an acid soil (lime requirement). The amount of lime needed to neutralize a soil must take account of the amount of acid forming ions on the colloids (exchangeable acidity), not just those in the soil water solution (free acidity).{{cite journal |last1=Farina |first1=Martin Patrick W. |last2=Sumner |first2=Malcolm E. |last3=Plank |first3=C. Owen |last4=Letzsch |first4=W. Stephen |year=1980 |title=Exchangeable aluminum and pH as indicators of lime requirement for corn |journal=Soil Science Society of America Journal |volume=44 |issue=5 |pages=1036–41 |url=https://www.researchgate.net/publication/250123873 |access-date=2 March 2025 |doi=10.2136/sssaj1980.03615995004400050033x |bibcode=1980SSASJ..44.1036F }} The addition of enough lime to neutralize the soil water solution will be insufficient to change the pH, as the acid forming cations stored on the soil colloids will tend to restore the original pH condition as they are pushed off those colloids by the calcium of the added lime.{{sfn|Donahue|Miller|Shickluna|1977|pp=119–120}}

{{Further|Soil conditioner}}

The resistance of soil to change in pH, as a result of the addition of acid or basic material, is a measure of the buffering capacity of a soil and (for a particular soil type) increases as the CEC increases. Hence, pure sand has almost no buffering ability, though soils high in colloids (whether mineral or organic) have high buffering capacity.{{cite journal |last1=Sposito |first1=Garrison |last2=Skipper |first2=Neal T. |last3=Sutton |first3=Rebecca |last4=Park |first4=Sun-Ho |last5=Soper |first5=Alan K. |last6=Greathouse |first6=Jeffery A. |year=1999 |title=Surface geochemistry of the clay minerals |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=96 |issue=7 |pages=3358–64 |doi=10.1073/pnas.96.7.3358 |pmid=10097044 |pmc=34275 |bibcode=1999PNAS...96.3358S |doi-access=free }} Buffering occurs by cation exchange and neutralisation. However, colloids are not the only regulators of soil pH. The role of carbonates should be underlined, too.{{cite web |last=Sparks |first=Donald L. |title=Acidic and basic soils: buffering |url=https://lawr.ucdavis.edu/classes/ssc102/Section8.pdf |publisher=University of California, Davis, Department of Land, Air, and Water Resources |location=Davis, California |access-date=9 March 2025 }} More generally, according to pH levels, several buffer systems take precedence over each other, from calcium carbonate buffer range to iron buffer range.{{cite book |last=Ulrich |first=Bernhard |title=Effects of accumulation of air pollutants in forest ecosystems |chapter=Soil acidity and its relations to acid deposition |date=1983 |chapter-url=https://archive.org/details/ulrich-1983 |pages=127–46 |edition=1st |editor-last1=Ulrich |editor-first1=Bernhard |editor-last2=Pankrath |editor-first2=Jürgen |publisher=D. Reidel Publishing Company |location=Dordrecht, The Netherlands |isbn=978-94-009-6985-8 |doi=10.1007/978-94-009-6983-4_10 |access-date=9 March 2025 }}

The addition of a small amount of highly basic aqueous ammonia to a soil will cause the ammonium to displace hydronium ions from the colloids, and the end product is water and colloidally fixed ammonium, but little permanent change overall in soil pH.

The addition of a small amount of lime, Ca(OH)₂, will displace hydronium ions from the soil colloids, causing the fixation of calcium to colloids and the evolution of CO₂ and water, with little permanent change in soil pH.

The above are examples of the buffering of soil pH. The general principal is that an increase in a particular cation in the soil water solution will cause that cation to be fixed to colloids (buffered) and a decrease in solution of that cation will cause it to be withdrawn from the colloid and moved into solution (buffered). The degree of buffering is often related to the CEC of the soil; the greater the CEC, the greater the buffering capacity of the soil.{{sfn|Donahue|Miller|Shickluna|1977|pp=120–121}}

{{main|Redox#Redox_reactions_in_soils}}

{{See also|Table of standard reduction potentials for half-reactions important in biochemistry}}

Soil chemical reactions involve some combination of proton and electron transfer. Oxidation occurs if there is a loss of electrons in the transfer process while reduction occurs if there is a gain of electrons. Reduction potential is measured in volts or millivolts. Soil microbial communities develop along electron transport chains, forming electrically conductive biofilms, and developing networks of bacterial nanowires.

Redox factors act on soil development, with redoximorphic color features providing critical information for soil interpretation. Understanding the redox gradient is important to managing carbon sequestration, bioremediation, wetland delineation, and soil-based microbial fuel cells.

{| class="wikitable sortable floatright"

|+ Plant nutrients, their chemical symbols, and the ionic forms common in soils and available for plant uptake{{sfn|Donahue|Miller|Shickluna|1977|p=125}}

|-

! Element !! Symbol !! Ion or molecule

|-

| Carbon || C || CO₂ (mostly through leaves)

|-

| Hydrogen || H || H⁺, H₂O (water)

|-

| Oxygen || O || O²⁻, OH⁻, {{chem|CO|3|2−}}, {{chem|SO|4|2−}}, CO₂

|-

| Phosphorus || P || {{chem|H|2|PO|4|−}}, {{chem|HPO|4|2−}} (phosphates)

|-

| Potassium || K || K⁺

|-

| Nitrogen || N || {{chem|NH|4|+}}, {{chem|NO|3|−}} (ammonium, nitrate)

|-

| Sulfur || S || {{chem|SO|4|2−}}

|-

| Calcium || Ca || Ca²⁺

|-

| Iron || Fe || Fe²⁺, Fe³⁺ (ferrous, ferric)

|-

| Magnesium || Mg || Mg²⁺

|-

| Boron || B || H₃BO₃, {{chem|H|2|BO|3|−}}, {{chem|B(OH)|4|−}}

|-

| Manganese || Mn || Mn²⁺

|-

| Copper || Cu || Cu²⁺

|-

| Zinc || Zn || Zn²⁺

|-

| Molybdenum || Mo || {{chem|MoO|4|2−}} (molybdate)

|-

| Chlorine || Cl || Cl⁻ (chloride)

|}

{{Main|Plant nutrients in soil|Plant nutrition|Soil pH#Effect of soil pH on plant growth}}

Seventeen elements or nutrients are essential for plant growth and reproduction. They are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), iron (Fe), boron (B), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), nickel (Ni) and chlorine (Cl).{{sfn|Dean|1957|p=80}}{{sfn|Russel|1957|pp=123–125}}{{cite book |title=The nature and properties of soils |year=1984 |edition=9th |last=Brady |first=Nyle C. |publisher=Macmillan Publishing Company |location=New York, New York |url=https://fr.1lib.sk/book/12005464/452990 |access-date=16 March 2025 |isbn=978-0029460306 }} Nutrients required for plants to complete their life cycle are considered essential nutrients. Nutrients that enhance the growth of plants but are not necessary to complete the plant's life cycle are considered non-essential. Except for carbon, hydrogen, and oxygen, which are supplied by carbon dioxide and water, and nitrogen, provided through nitrogen fixation, the nutrients derive originally from the mineral component of the soil. The Law of the Minimum expresses that when the available form of a nutrient is not in enough proportion in the soil solution, other nutrients cannot be taken up at an optimum rate by a plant.{{cite journal |last1=Van der Ploeg |first1=Rienk R. |last2=Böhm |first2=Wolfgang |last3=Kirkham |first3=Mary Beth |year=1999 |title=On the origin of the theory of mineral nutrition of plants and the Law of the Minimum |journal=Soil Science Society of America Journal |volume=63 |issue=5 |pages=1055–62 |doi=10.2136/sssaj1999.6351055x |citeseerx=10.1.1.475.7392 |bibcode=1999SSASJ..63.1055V |doi-access=free }} A particular nutrient ratio of the soil solution is thus mandatory for optimizing plant growth, a value which might differ from nutrient ratios calculated from plant composition.{{cite journal |last1=Knecht |first1=Magnus F. |last2=Göransson |first2=Anders |year=2004 |title=Terrestrial plants require nutrients in similar proportions |journal=Tree Physiology |volume=24 |issue=4 |pages=447–60 |doi=10.1093/treephys/24.4.447 |pmid=14757584 |url=https://fr.1lib.sk/book/72174734/946166 |access-date=16 March 2025 }}

Plant uptake of nutrients can only proceed when present in a plant-available form. In most situations, nutrients are absorbed in an ionic form from (or together with) soil water. Although minerals are the origin of most nutrients, and the bulk of most nutrient elements in the soil is held in crystalline form within primary and secondary minerals, they weather too slowly to support rapid plant growth. For example, the application of finely ground minerals, feldspar and apatite, to soil seldom provides the necessary amounts of potassium and phosphorus at a rate sufficient for good plant growth, as most of the nutrients remain bound in the crystals of those minerals.{{sfn|Dean|1957|pp=80–81}} However, plants are able to stimulate mineral weathering, and thus the availability of mineral-bound nutrients, through various processes, both direct (e.g. weathering agents, biogenic minerals) and indirect (e.g. mycorrhizal fungi, rhizosphere bacteria).{{cite journal |last1=Kelly |first1=Eugene F. |last2=Chadwick |first2=Oliver A. |last3=Hiinski |first3=Thomas E. |year=1998 |title=The effect of plants on mineral weathering |journal=Biogeochemistry |volume=42 |pages=21–53 |doi=10.1023/A:1005919306687 |url=https://www.academia.edu/87628533 |access-date=23 March 2025 }}{{cite journal |last1=Calvaruso |first1=Christophe |last2=Turpault |first2=Marie-Pierre |last3=Frey-Klett |first3=Pascale |year=February 2006 |title=Root-associated bacteria contribute to mineral weathering and to mineral nutrition in trees: a budgeting analysis |journal=Applied and Environmental Microbiology |volume=72 |issue=2 |pages=1258–66 |doi=10.1128/AEM.72.2.1258-1266.2006 |url=https://journals.asm.org/doi/pdf/10.1128/aem.72.2.1258-1266.2006 |access-date=23 March 2025 }}{{cite journal |last1=Van Schöll |first1=Laura |last2=Kuyper |first2=Thomas W. |last3=Smits |first3=Mark M. |last4=Landeweert |first4=Renske |last5=Hoffland |first5=Ellis |last6=Van Breemen |first6=Nico |year=21 December 2007 |title=Rock-eating mycorrhizas: their role in plant nutrition and biogeochemical cycles |journal=Plant and Soil |volume=303 |pages=35–47 |doi=10.1007/s11104-007-9513-0 |url=https://core.ac.uk/download/pdf/29259822.pdf |access-date=23 March 2025 }}

The nutrients adsorbed onto the surfaces of clay colloids and soil organic matter provide a more accessible reservoir of many plant nutrients (e.g., K, Ca, Mg, P, Zn). As plants absorb the nutrients from the soil water, the soluble pool is replenished from the surface-bound pool. The decomposition of soil organic matter by microorganisms is another mechanism whereby the soluble pool of nutrients is replenished – this is important for the supply of plant-available N, S, P, and B from soil.{{cite book |chapter-url=https://www.fao.org/fileadmin/templates/soilbiodiversity/Downloadable_files/fpnb16.pdf#page=61.14 |title=Plant nutrition for food security: a guide for integrated nutrient management |last1=Roy |first1=R. N. |last2=Finck |first2=Arnold |last3=Blair |first3=Graeme J. |last4=Tandon |first4=Hari Lal Singh |publisher=Food and Agriculture Organization of the United Nations |year=2006 |isbn=978-92-5-105490-1 |location=Rome, Italy |pages=43–90 |chapter=Soil fertility and crop production |access-date=30 March 2025 }}

Gram for gram, the capacity of humus to hold nutrients and water is far greater than that of clay minerals, most of the soil cation exchange capacity arising from charged carboxylic groups on organic matter.{{cite journal |last1=Parfitt |first1=Roger L. |last2=Giltrap |first2=Donna J. |last3=Whitton |first3=Joe S. |year=1995 |title=Contribution of organic matter and clay minerals to the cation exchange capacity of soil |journal=Communications in Soil Science and Plant Analysis |volume=26 |issue=9–10 |pages=1343–55 |url=https://www.researchgate.net/publication/249073571 |doi=10.1080/00103629509369376 |bibcode=1995CSSPA..26.1343P |access-date=30 March 2025 }} However, despite the remarkable capacity of humus to retain water once water-soaked, its high hydrophobicity decreases its wettability once dry.{{cite journal |last1=Hajnos |first1=Mieczyslaw |last2=Jozefaciuk |first2=Grzegorz |last3=Sokołowska |first3=Zofia |last4=Greiffenhagen |first4=Andreas |last5=Wessolek |first5=Gerd |year=2003 |title=Water storage, surface, and structural properties of sandy forest humus horizons |journal=Journal of Plant Nutrition and Soil Science |volume=166 |issue=5 |pages=625–34 |url=https://www.researchgate.net/publication/229970348 |doi=10.1002/jpln.200321161 |bibcode=2003JPNSS.166..625H |access-date=30 March 2025 }} Small amounts of humus may remarkably increase the soil's capacity to promote plant growth.{{sfn|Donahue|Miller|Shickluna|1977|pp=123–131}}

{{main|Soil organic matter}}{{Overly detailed|section|details=details could be moved into main article|date=April 2021}}

The organic material in soil is made up of organic compounds and includes plant, animal and microbial material, both living and dead. A typical soil has a biomass composition of 70% microorganisms, 22% macrofauna, and 8% roots. The living component of an acre of soil may include 900 lb of earthworms, 2400 lb of fungi, 1500 lb of bacteria, 133 lb of protozoa and 890 lb of arthropods and algae.{{cite journal |last1=Pimentel |first1=David |last2=Harvey |first2=Celia |last3=Resosudarmo |first3=Pradnja |last4=Sinclair |first4=K. |last5=Kurz |first5=D. |last6=McNair |first6=M. |last7=Crist |first7=S. |last8=Shpritz |first8=L. |last9=Fitton |first9=L. |last10=Saffouri |first10=R. |last11=Blair |first11=R. |year=1995 |title=Environmental and economic costs of soil erosion and conservation benefits |journal=Science |volume=267 |issue=5201 |pages=1117–23 |url=https://www.academia.edu/9512072 |doi=10.1126/science.267.5201.1117 |pmid=17789193 |bibcode=1995Sci...267.1117P |s2cid=11936877 |access-date=6 April 2025 |archive-url=https://web.archive.org/web/20161213065558/http://www.rachel.org/files/document/Environmental_and_Economic_Costs_of_Soil_Erosi.pdf |archive-date=13 December 2016 |url-status=live }}

A few percent of the soil organic matter, with small residence time, consists of the microbial biomass and metabolites of bacteria, molds, and actinomycetes that work to break down the dead organic matter.{{cite journal |last1=Schnürer |first1=Johan |last2=Clarholm |first2=Marianne |last3=Rosswall |first3=Thomas |year=1985 |title=Microbial biomass and activity in an agricultural soil with different organic matter contents |journal=Soil Biology and Biochemistry |volume=17 |issue=5 |pages=611–18 |url=https://www.academia.edu/20647751 |doi=10.1016/0038-0717(85)90036-7 |bibcode=1985SBiBi..17..611S |access-date=6 April 2025 }}{{cite journal |last=Sparling |first=Graham P. |year=1992 |title=Ratio of microbial biomass carbon to soil organic carbon as a sensitive indicator of changes in soil organic matter |journal=Australian Journal of Soil Research |volume=30 |issue=2 |pages=195–207 |url=https://www.researchgate.net/publication/248884528 |doi=10.1071/SR9920195 |access-date=6 April 2025 }} Were it not for the action of these micro-organisms, the entire carbon dioxide part of the atmosphere would be sequestered as organic matter in the soil. However, in the same time soil microbes contribute to carbon sequestration in the topsoil through the formation of stable humus.{{cite journal |last1=Varadachari |first1=Chandrika |last2=Ghosh |first2=Kunal |year=1984 |title=On humus formation |journal=Plant and Soil |volume=77 |issue=2 |pages=305–13 |doi=10.1007/BF02182933 |bibcode=1984PlSoi..77..305V |s2cid=45102095 |url=https://www.researchgate.net/publication/225528442 |access-date=6 April 2025 }} In the aim to sequester more carbon in the soil for alleviating the greenhouse effect it would be more efficient in the long-term to stimulate humification than to decrease litter decomposition.{{cite journal |last=Prescott |first=Cindy E. |year=2010 |title=Litter decomposition: what controls it and how can we alter it to sequester more carbon in forest soils? |journal=Biogeochemistry |volume=101 |issue=1 |pages=133–49 |doi=10.1007/s10533-010-9439-0 |bibcode=2010Biogc.101..133P |s2cid=93834812 |url=https://www.researchgate.net/publication/226936910 |access-date=6 April 2025 }}

The main part of soil organic matter is a complex assemblage of small organic molecules, collectively called humus or humic substances. The use of these terms, which do not rely on a clear chemical classification, has been considered as obsolete.{{cite journal |last1=Lehmann |first1=Johannes |last2=Kleber |first2=Markus |year=2015 |title=The contentious nature of soil organic matter |journal=Nature |volume=528 |issue=7580 |pages=60–68 |url=http://www.css.cornell.edu/faculty/lehmann/publ/Nature%20528,%2060-68,%202015%20Lehmann.pdf |doi=10.1038/nature16069 |pmid=26595271 |bibcode=2015Natur.528...60L |s2cid=205246638 |access-date=13 April 2025 }} Other studies showed that the classical notion of molecule is not convenient for humus, which escaped most attempts done over two centuries to resolve it in unit components, but still is chemically distinct from polysaccharides, lignins and proteins.{{cite journal |last=Piccolo |first=Alessandro |year=2002 |title=The supramolecular structure of humic substances: a novel understanding of humus chemistry and implications in soil science |journal=Advances in Agronomy |volume=75 |pages=57–134 |url=https://www.researchgate.net/publication/222526145 |doi=10.1016/S0065-2113(02)75003-7 |isbn=9780120007936 |access-date=13 April 2025 }}

Most living things in soils, including plants, animals, bacteria, and fungi, are dependent on organic matter for nutrients and/or energy. Soils have organic compounds in varying degrees of decomposition, the rate of which is dependent on the temperature, soil moisture, and aeration. Bacteria and fungi feed on the raw organic matter, which are fed upon by protozoa, which in turn are fed upon by nematodes, annelids and arthropods, themselves able to consume and transform raw or humified organic matter. This has been called the soil food web, through which all organic matter is processed as in a digestive system.{{cite journal |last=Scheu |first=Stefan |year=2002 |title=The soil food web: structure and perspectives |journal=European Journal of Soil Biology |volume=38 |issue=1 |pages=11–20 |url=https://www.researchgate.net/publication/263041521 |doi=10.1016/S1164-5563(01)01117-7 |bibcode=2002EJSB...38...11S |access-date=13 April 2025 }} Organic matter holds soils open, allowing the infiltration of air and water, and may hold as much as twice its weight in water. Many soils, including desert and rocky-gravel soils, have little or no organic matter. Soils that are all organic matter, such as peat (histosols), are infertile.{{Cite book |last=Foth |first=Henry D. |year=1984 |title=Fundamentals of soil science |edition=8th |page=139 |url=http://base.dnsgb.com.ua/files/book/Agriculture/Soil/Fundamentals-of-Soil-Science.pdf |isbn=978-0471522799 |publisher=Wiley |location=New York, New York |access-date=13 April 2025 |archive-date=12 November 2020 |archive-url=https://web.archive.org/web/20201112034423/http://base.dnsgb.com.ua/files/book/Agriculture/Soil/Fundamentals-of-Soil-Science.pdf |url-status=live }} In its earliest stage of decomposition, the original organic material is often called raw organic matter. The final stage of decomposition is called humus.

In grassland, much of the organic matter added to the soil is from the deep, fibrous, grass root systems. By contrast, tree leaves falling on the forest floor are the principal source of soil organic matter in the forest. Another difference is the frequent occurrence in the grasslands of fires that destroy large amounts of aboveground material but stimulate even greater contributions from roots. Also, the much greater acidity under any forests inhibits the action of certain soil organisms that otherwise would mix much of the surface litter into the mineral soil. As a result, the soils under grasslands generally develop a thicker A horizon with a deeper distribution of organic matter than in comparable soils under forests, which characteristically store most of their organic matter in the forest floor (O horizon) and thin A horizon.{{cite journal |last=Ponge |first=Jean-François |year=2003 |title=Humus forms in terrestrial ecosystems: a framework to biodiversity |journal=Soil Biology and Biochemistry |volume=35 |issue=7 |pages=935–45 |doi=10.1016/S0038-0717(03)00149-4 |bibcode=2003SBiBi..35..935P |url=https://www.academia.edu/45579598 |url-status=live |archive-url=https://web.archive.org/web/20160129153903/https://www.researchgate.net/publication/222567430 |archive-date=29 January 2016 |df=dmy-all |citeseerx=10.1.1.467.4937 |s2cid=44160220 |access-date=13 April 2025 }}

Humus refers to organic matter that has been decomposed by soil microflora and fauna to the point where it is resistant to further breakdown.{{cite journal |last=Ponge |first=Jean-François |date=August 2022 |title=Humus: dark side of life or intractable “aether”? |journal=Pedosphere |volume=32 |issue=4 |pages=660–64 |url=https://www.researchgate.net/publication/360175852 |doi=10.1016/S1002-0160(21)60013-9 |access-date=13 April 2025 }} Humus usually constitutes only five percent of the soil or less by volume, but it is an essential source of nutrients and adds important textural qualities crucial to soil health and plant growth.{{cite web |url=https://humates.com/wp-content/uploads/2020/04/ORGANICMATTERPettit.pdf |last=Pettit |first=Robert E. |title=Organic matter, humus, humate, humic acid, fulvic acid and humin: their importance in soil fertility and plant health |access-date=13 April 2025 }} Humus also feeds arthropods, termites and earthworms which further improve the soil.{{cite journal |last1=Ji |first1=Rong |last2=Kappler |first2=Andreas |last3=Brune |first3=Andreas |year=2000 |title=Transformation and mineralization of synthetic ¹⁴C-labeled humic model compounds by soil-feeding termites |journal=Soil Biology and Biochemistry |volume=32 |issue=8–9 |pages=1281–1291 |doi=10.1016/S0038-0717(00)00046-8 |citeseerx=10.1.1.476.9400 }} The end product, humus, is suspended in colloidal form in the soil solution and forms a weak acid that can attack silicate minerals by chelating their iron and aluminum atoms.{{cite book |last1=Drever |first1=James I. |last2=Vance |first2=George F. |title=Organic Acids in Geological Processes |chapter=Role of Soil Organic Acids in Mineral Weathering Processes |year=1994 |doi=10.1007/978-3-642-78356-2_6 |editor-last1=Pittman |editor-first1=Edward D. |editor-last2=Lewan |editor-first2=Michael D. |publisher=Springer |location=Berlin, Germany |pages=138–161 |isbn=978-3-642-78356-2 |chapter-url=https://link.springer.com/content/pdf/10.1007%2F978-3-642-78356-2_6.pdf |access-date=11 July 2021}} Humus has a high cation and anion exchange capacity that on a dry weight basis is many times greater than that of clay colloids. It also acts as a buffer, like clay, against changes in pH and soil moisture.{{cite book |last=Piccolo |first=Alessandro |year=1996 |chapter=Humus and soil conservation |doi=10.1016/B978-044481516-3/50006-2 |title=Humic substances in terrestrial ecosystems |editor-first=Alessandro |editor-last=Piccolo |publisher= Elsevier |location=Amsterdam, the Netherlands |pages=225–264 |isbn=978-0-444-81516-3 |chapter-url=https://www.researchgate.net/publication/281451183 |access-date=11 July 2021}}

Humic acids and fulvic acids, which begin as raw organic matter, are important constituents of humus. After the death of plants, animals, and microbes, microbes begin to feed on the residues through their production of extra-cellular soil enzymes, resulting finally in the formation of humus.{{cite journal |last1=Varadachari |first1=Chandrika |last2=Ghosh |first2=Kunal |year=1984 |title=On humus formation |journal=Plant and Soil |volume=77 |issue=2 |pages=305–313 |url=https://www.researchgate.net/publication/225528442 |doi=10.1007/BF02182933 |bibcode=1984PlSoi..77..305V |s2cid=45102095 |access-date=11 July 2021}} As the residues break down, only molecules made of aliphatic and aromatic hydrocarbons, assembled and stabilized by oxygen and hydrogen bonds, remain in the form of complex molecular assemblages collectively called humus. Humus is never pure in the soil, because it reacts with metals and clays to form complexes which further contribute to its stability and to soil structure. Although the structure of humus has in itself few nutrients (with the exception of constitutive metals such as calcium, iron and aluminum) it is able to attract and link, by weak bonds, cation and anion nutrients that can further be released into the soil solution in response to selective root uptake and changes in soil pH, a process of paramount importance for the maintenance of fertility in tropical soils.{{cite journal |last1=Mendonça |first1=Eduardo S. |last2=Rowell |first2=David L. |year=1996 |title=Mineral and organic fractions of two oxisols and their influence on effective cation-exchange capacity |journal=Soil Science Society of America Journal |volume=60 |issue=6 |pages=1888–1892 |url=https://www.researchgate.net/publication/250128642 |doi=10.2136/sssaj1996.03615995006000060038x |bibcode=1996SSASJ..60.1888M |access-date=11 July 2021}}

Lignin is resistant to breakdown and accumulates within the soil. It also reacts with proteins,{{cite journal |last1=Heck |first1=Tobias |last2=Faccio |first2=Greta |last3=Richter |first3=Michael |last4=Thöny-Meyer |first4=Linda |year=2013 |title=Enzyme-catalyzed protein crosslinking |journal=Applied Microbiology and Biotechnology |volume=97 |issue=2 |pages=461–475 |url=https://www.researchgate.net/publication/233769618 |doi=10.1007/s00253-012-4569-z |pmid=23179622 |pmc=3546294 |access-date=11 July 2021}} which further increases its resistance to decomposition, including enzymatic decomposition by microbes.{{cite journal |last1=Lynch |first1=D. L. |last2=Lynch |first2=C. C. |year=1958 |title=Resistance of protein–lignin complexes, lignins and humic acids to microbial attack |journal=Nature |volume=181 |issue=4621 |pages=1478–1479 |url=https://www.nature.com/articles/1811478a0.pdf |doi=10.1038/1811478a0 |pmid=13552710 |bibcode=1958Natur.181.1478L |s2cid=4193782 |access-date=11 July 2021}} Fats and waxes from plant matter have still more resistance to decomposition and persist in soils for thousand years, hence their use as tracers of past vegetation in buried soil layers.{{cite journal |last1=Dawson |first1=Lorna A. |last2=Hillier |first2=Stephen |year=2010 |title=Measurement of soil characteristics for forensic applications |journal=Surface and Interface Analysis |volume=42 |issue=5 |pages=363–377 |url=https://people.ok.ubc.ca/robrien/soil%20characteristics.pdf |doi=10.1002/sia.3315 |s2cid=54213404 |access-date=18 July 2021 |archive-date=8 May 2021 |archive-url=https://web.archive.org/web/20210508065204/https://people.ok.ubc.ca/robrien/soil%20characteristics.pdf |url-status=dead }} Clay soils often have higher organic contents that persist longer than soils without clay as the organic molecules adhere to and are stabilised by the clay.{{cite journal |last1=Manjaiah |first1=K.M. |last2=Kumar |first2=Sarvendra |last3=Sachdev |first3=M. S. |last4=Sachdev |first4=P. |last5=Datta |first5=S. C. |year=2010 |title=Study of clay–organic complexes |journal=Current Science |volume=98 |issue=7 |pages=915–921 |url=https://www.researchgate.net/publication/228867334 |access-date=18 July 2021}} Proteins normally decompose readily, to the exception of scleroproteins, but when bound to clay particles they become more resistant to decomposition.{{cite journal |last=Theng |first=Benny K.G. |year=1982 |title=Clay-polymer interactions: summary and perspectives |journal=Clays and Clay Minerals |volume=30 |issue=1 |pages=1–10 |doi=10.1346/CCMN.1982.0300101 |bibcode=1982CCM....30....1T |citeseerx=10.1.1.608.2942 |s2cid=98176725 }} As for other proteins clay particles absorb the enzymes exuded by microbes, decreasing enzyme activity while protecting extracellular enzymes from degradation.{{cite journal |last1=Tietjen |first1=Todd |last2=Wetzel |first2=Robert G. |year=2003 |title=Extracellular enzyme-clay mineral complexes: enzyme adsorption, alteration of enzyme activity, and protection from photodegradation |journal=Aquatic Ecology |volume=37 |issue=4 |pages=331–339 |doi=10.1023/B:AECO.0000007044.52801.6b |bibcode=2003AqEco..37..331T |s2cid=6930871 |url=http://www.vliz.be/imisdocs/publications/54440.pdf |access-date=18 July 2021}} The addition of organic matter to clay soils can render that organic matter and any added nutrients inaccessible to plants and microbes for many years.{{cite journal |last1=Tahir |first1=Shermeen |last2=Marschner |first2=Petra |year=2017 |title=Clay addition to sandy soil: influence of clay type and size on nutrient availability in sandy soils amended with residues differing in C/N ratio |journal=Pedosphere |volume=27 |issue=2 |pages=293–305 |url=https://www.researchgate.net/publication/314221508 |doi=10.1016/S1002-0160(17)60317-5 |bibcode=2017Pedos..27..293T |access-date=18 July 2021}} A study showed increased soil fertility following the addition of mature compost to a clay soil.{{cite journal |last1=Melero |first1=Sebastiana |last2=Madejón |first2=Engracia |last3=Ruiz |first3=Juan Carlos |last4=Herencia |first4=Juan Francisco |year=2007 |title=Chemical and biochemical properties of a clay soil under dryland agriculture system as affected by organic fertilization |journal=European Journal of Agronomy |volume=26 |issue=3 |pages=327–334 |url=https://coek.info/pdf-chemical-and-biochemical-properties-of-a-clay-soil-under-dryland-agriculture-sys.html |doi=10.1016/j.eja.2006.11.004 |bibcode=2007EuJAg..26..327M |access-date=18 July 2021}} High soil tannin content can cause nitrogen to be sequestered as resistant tannin-protein complexes.{{cite journal |last1=Joanisse |first1=Gilles D. |last2=Bradley |first2=Robert L. |last3=Preston |first3=Caroline M. |last4=Bending |first4=Gary D. |title=Sequestration of soil nitrogen as tannin–protein complexes may improve the competitive ability of sheep laurel (Kalmia angustifolia) relative to black spruce (Picea mariana) |journal=New Phytologist |year=2009 |volume=181 |pages=187–198 |doi=10.1111/j.1469-8137.2008.02622.x |issue=1 |pmid=18811620 |doi-access=free|bibcode=2009NewPh.181..187J }}{{cite journal |last1=Fierer |first1=Noah |last2=Schimel |first2=Joshua P. |last3=Cates |first3=Rex G. |last4=Zou |first4=Jiping |title=Influence of balsam poplar tannin fractions on carbon and nitrogen dynamics in Alaskan taiga floodplain soils |journal=Soil Biology and Biochemistry |year=2001 |volume=33 |pages=1827–1839 |doi=10.1016/S0038-0717(01)00111-0 |issue=12–13 |bibcode=2001SBiBi..33.1827F |url=https://www.academia.edu/12814037 |access-date=18 July 2021}}

Humus formation is a process dependent on the amount of plant material added each year and the type of base soil. Both are affected by climate and the type of organisms present. Soils with humus can vary in nitrogen content but typically have 3 to 6 percent nitrogen. Raw organic matter, as a reserve of nitrogen and phosphorus, is a vital component affecting soil fertility. Humus also absorbs water, and expands and shrinks between dry and wet states to a higher extent than clay, increasing soil porosity.{{cite journal |last1=Peng |first1=Xinhua |last2=Horn |first2=Rainer |title=Anisotropic shrinkage and swelling of some organic and inorganic soils |journal=European Journal of Soil Science |year=2007 |volume=58 |issue=1 |pages=98–107 |doi=10.1111/j.1365-2389.2006.00808.x |bibcode=2007EuJSS..58...98P |doi-access=free}} Humus is less stable than the soil's mineral constituents, as it is reduced by microbial decomposition, and over time its concentration diminishes without the addition of new organic matter. However, humus in its most stable forms may persist over centuries if not millennia.{{cite journal |last1=Wang |first1=Yang |last2=Amundson |first2=Ronald |last3=Trumbmore |first3=Susan |title=Radiocarbon dating of soil organic matter |journal=Quaternary Research |year=1996 |volume=45 |issue=3 |pages=282–288 |doi=10.1006/qres.1996.0029 |bibcode=1996QuRes..45..282W |s2cid=73640995 |url=https://escholarship.org/content/qt6b14h4bv/qt6b14h4bv.pdf |access-date=18 July 2021}} Charcoal is a source of highly stable humus, called black carbon,{{cite journal |last1=Brodowski |first1=Sonja |last2=Amelung |first2=Wulf |last3=Haumaier |first3=Ludwig |last4=Zech |first4=Wolfgang |title=Black carbon contribution to stable humus in German arable soils |journal=Geoderma |year=2007 |volume=139 |issue=1–2 |pages=220–228 |doi=10.1016/j.geoderma.2007.02.004 |bibcode=2007Geode.139..220B |url=https://www.academia.edu/33858429 |access-date=18 July 2021}} which had been used traditionally to improve the fertility of nutrient-poor tropical soils. This very ancient practice, as ascertained in the genesis of Amazonian dark earths, has been renewed and became popular under the name of biochar. It has been suggested that biochar could be used to sequester more carbon in the fight against the greenhouse effect.{{cite journal |last1=Criscuoli |first1=Irene |last2=Alberti |first2=Giorgio |last3=Baronti |first3=Silvia |last4=Favilli |first4=Filippo |last5=Martinez |first5=Cristina |last6=Calzolari |first6=Costanza |last7=Pusceddu |first7=Emanuela |last8=Rumpel |first8=Cornelia |last9=Viola |first9=Roberto |last10=Miglietta |first10=Franco |title=Carbon sequestration and fertility after centennial time scale incorporation of charcoal into soil |journal=PLOS ONE |year=2014 |volume=9 |issue=3 |pages=e91114 |doi=10.1371/journal.pone.0091114 |pmc=3948733 |pmid=24614647|bibcode=2014PLoSO...991114C |doi-access=free}}

The production, accumulation and degradation of organic matter are greatly dependent on climate. For example, when a thawing event occurs, the flux of soil gases with atmospheric gases is significantly influenced.{{cite journal |last1=Kim |first1=Dong Jim |last2=Vargas |first2=Rodrigo |last3=Bond-Lamberty |first3=Ben |last4=Turetsky |first4=Merritt R. |title=Effects of soil rewetting and thawing on soil gas fluxes: a review of current literature and suggestions for future research |journal=Biogeosciences |year=2012 |volume=9 |issue=7 |pages=2459–2483 |doi=10.5194/bg-9-2459-2012 |bibcode=2012BGeo....9.2459K |url=https://www.researchgate.net/publication/307827983 |access-date=3 October 2021|doi-access=free }} Temperature, soil moisture and topography are the major factors affecting the accumulation of organic matter in soils. Organic matter tends to accumulate under wet or cold conditions where decomposer activity is impeded by low temperature{{cite journal |last1=Wagai |first1=Rota |last2=Mayer |first2=Lawrence M. |last3=Kitayama |first3=Kanehiro |last4=Knicker |first4=Heike |year=2008 |title=Climate and parent material controls on organic matter storage in surface soils: a three-pool, density-separation approach |journal=Geoderma |volume=147 |issue=1–2 |pages=23–33 |doi=10.1016/j.geoderma.2008.07.010 |bibcode=2008Geode.147...23W |url=https://www.academia.edu/20165844 |access-date=25 July 2021 |hdl=10261/82461 |hdl-access=free}} or excess moisture which results in anaerobic conditions.{{cite journal |last1=Minayeva |first1=Tatiana Y. |last2=Trofimov |first2=Sergey Ya. |last3=Chichagova |first3=Olga A. |last4=Dorofeyeva |first4=E. I. |last5=Sirin |first5=Andrey A. |last6=Glushkov |first6=Igor V. |last7=Mikhailov |first7=N. D. |last8=Kromer |first8=Bernd |year=2008 |title=Carbon accumulation in soils of forest and bog ecosystems of southern Valdai in the Holocene |journal=Biology Bulletin |volume=35 |issue=5 |pages=524–532 |doi=10.1134/S1062359008050142 |bibcode=2008BioBu..35..524M |s2cid=40927739 |url=https://www.researchgate.net/publication/225229436 |access-date=25 July 2021}} Conversely, excessive rain and high temperatures of tropical climates enables rapid decomposition of organic matter and leaching of plant nutrients. Forest ecosystems on these soils rely on efficient recycling of nutrients and plant matter by the living plant and microbial biomass to maintain their productivity, a process which is disturbed by human activities.{{cite journal |last1=Vitousek |first1=Peter M. |last2=Sanford |first2=Robert L. |title=Nutrient cycling in moist tropical forest |journal=Annual Review of Ecology and Systematics |year=1986 |volume=17 |issue=1 |pages=137–167 |doi=10.1146/annurev.es.17.110186.001033 |bibcode=1986AnRES..17..137V |s2cid=55212899 |url=https://www.researchgate.net/publication/234150505 |access-date=25 July 2021}} Excessive slope, in particular in the presence of cultivation for the sake of agriculture, may encourage the erosion of the top layer of soil which holds most of the raw organic material that would otherwise eventually become humus.{{cite journal |last1=Rumpel |first1=Cornelia |last2=Chaplot |first2=Vincent |last3=Planchon |first3=Olivier |last4=Bernadou |first4=J. |last5=Valentin |first5=Christian |last6=Mariotti |first6=André |title=Preferential erosion of black carbon on steep slopes with slash and burn agriculture |journal=Catena |year=2006 |volume=65 |issue=1 |pages=30–40 |url=https://www.academia.edu/14788543 |doi=10.1016/j.catena.2005.09.005 |bibcode=2006Caten..65...30R |access-date=25 July 2021}}

{{Pie chart

|caption = Typical types and percentages of plant residue components

|value1 = 45

|label1 = Cellulose

|value2 = 20

|label2 = Lignin

|value3 = 18

|label3 = Hemicellulose

|value4 = 8

|label4 = Protein

|value5 = 5

|label5 = Sugars and starches

|value6 = 2

|label6 = Fats and waxes

}}

Cellulose and hemicellulose undergo fast decomposition by fungi and bacteria, with a half-life of 12–18 days in a temperate climate.{{cite book|last1=Paul |first1=Eldor A. |last2=Paustian |first2=Keith H. |last3=Elliott |first3=E. T. |last4=Cole |first4=C. Vernon |title=Soil organic matter in temperate agroecosystems: long-term experiments in North America |date=1997 |publisher=CRC Press |location=Boca Raton, Florida |isbn=978-0-8493-2802-2 |page=80}} Brown rot fungi can decompose the cellulose and hemicellulose, leaving the lignin and phenolic compounds behind. Starch, which is an energy storage system for plants, undergoes fast decomposition by bacteria and fungi. Lignin consists of polymers composed of 500 to 600 units with a highly branched, amorphous structure, linked to cellulose, hemicellulose and pectin in plant cell walls. Lignin undergoes very slow decomposition, mainly by white rot fungi and actinomycetes; its half-life under temperate conditions is about six months.

{{Main|Soil horizon}}

A horizontal layer of the soil, whose physical features, composition and age are distinct from those above and beneath, is referred to as a soil horizon. The naming of a horizon is based on the type of material of which it is composed. Those materials reflect the duration of specific processes of soil formation. They are labelled using a shorthand notation of letters and numbers which describe the horizon in terms of its colour, size, texture, structure, consistency, root quantity, pH, voids, boundary characteristics and presence of nodules or concretions.{{cite web |url=https://soilsofcanada.ca/soil-formation/horizons.php |title=Horizons |website=Soils of Canada |access-date=1 August 2021 |archive-url=https://web.archive.org/web/20190922153041/https://soilsofcanada.ca/soil-formation/horizons.php |archive-date=22 September 2019 |url-status=live}} No soil profile has all the major horizons. Some, called entisols, may have only one horizon or are currently considered as having no horizon, in particular incipient soils from unreclaimed mining waste deposits,{{cite journal |last1=Frouz |first1=Jan |last2=Prach |first2=Karel |last3=Pizl |first3=Václav |last4=Háněl |first4=Ladislav |last5=Starý |first5=Josef |last6=Tajovský |first6=Karel |last7=Materna |first7=Jan |last8=Balík |first8=Vladimír |last9=Kalčík |first9=Jiří |last10=Řehounková |first10=Klára |year=2008 |title=Interactions between soil development, vegetation and soil fauna during spontaneous succession in post mining sites |journal=European Journal of Soil Biology |volume=44 |issue=1 |pages=109–121 |url=https://www.researchgate.net/publication/223699609 |doi=10.1016/j.ejsobi.2007.09.002 |bibcode=2008EJSB...44..109F |access-date=1 August 2021}} moraines,{{cite journal |last1=Kabala |first1=Cezary |last2=Zapart |first2=Justyna |year=2012 |title=Initial soil development and carbon accumulation on moraines of the rapidly retreating Werenskiold Glacier, SW Spitsbergen, Svalbard archipelago |journal=Geoderma |volume=175–176 |pages=9–20 |url=https://www.academia.edu/31221217 |doi=10.1016/j.geoderma.2012.01.025 |bibcode=2012Geode.175....9K |access-date=1 August 2021}} volcanic cones{{cite journal |last1=Ugolini |first1=Fiorenzo C. |last2=Dahlgren |first2=Randy A. |year=2002 |title=Soil development in volcanic ash |journal=Global Environmental Research |volume=6 |issue=2 |pages=69–81 |url=http://www.airies.or.jp/attach.php/6a6f75726e616c5f30362d32656e67/save/0/0/06_2-09.pdf |access-date=1 August 2021}} sand dunes or alluvial terraces.{{cite journal |last=Huggett |first=Richard J. |year=1998 |title=Soil chronosequences, soil development, and soil evolution: a critical review |journal=Catena |volume=32 |issue=3 |pages=155–172 |url=https://www.academia.edu/2116704 |doi=10.1016/S0341-8162(98)00053-8 |bibcode=1998Caten..32..155H |access-date=1 August 2021}} Upper soil horizons may be lacking in truncated soils following wind or water ablation, with concomitant downslope burying of soil horizons, a natural process aggravated by agricultural practices such as tillage.{{cite journal |last1=De Alba |first1=Saturnio |last2=Lindstrom |first2=Michael |last3=Schumacher |first3=Thomas E. |last4=Malo |first4=Douglas D. |year=2004 |title=Soil landscape evolution due to soil redistribution by tillage: a new conceptual model of soil catena evolution in agricultural landscapes |journal=Catena |volume=58 |issue=1 |pages=77–100 |url=https://www.academia.edu/22300477 |doi=10.1016/j.catena.2003.12.004 |bibcode=2004Caten..58...77D |access-date=1 August 2021}} The growth of trees is another source of disturbance, creating a micro-scale heterogeneity which is still visible in soil horizons once trees have died.{{cite journal |last1=Phillips |first1=Jonathan D. |last2=Marion |first2=Daniel A. |year=2004 |title=Pedological memory in forest soil development |journal=Forest Ecology and Management |volume=188 |issue=1 |pages=363–380 |url=https://www.srs.fs.usda.gov/pubs/ja/ja_phillips004.pdf |doi=10.1016/j.foreco.2003.08.007 |bibcode=2004ForEM.188..363P |access-date=1 August 2021}} By passing from a horizon to another, from the top to the bottom of the soil profile, one goes back in time, with past events registered in soil horizons like in sediment layers. Sampling pollen, testate amoebae and plant remains in soil horizons may help to reveal environmental changes (e.g. climate change, land use change) which occurred in the course of soil formation.{{cite journal |last1=Mitchell |first1=Edward A.D. |last2=Van der Knaap |first2=Willem O. |last3=Van Leeuwen |first3=Jacqueline F.N. |last4=Buttler |first4=Alexandre |last5=Warner |first5=Barry G. |last6=Gobat |first6=Jean-Michel |year=2001 |title=The palaeoecological history of the Praz-Rodet bog (Swiss Jura) based on pollen, plant macrofossils and testate amoebae(Protozoa) |journal=The Holocene |volume=11 |issue=1 |pages=65–80 |url=https://www.academia.edu/31915005 |doi=10.1191/095968301671777798 |bibcode=2001Holoc..11...65M |s2cid=131032169 |access-date=1 August 2021}} Soil horizons can be dated by several methods such as radiocarbon, using pieces of charcoal provided they are of enough size to escape pedoturbation by earthworm activity and other mechanical disturbances.{{cite journal |last=Carcaillet |first=Christopher |year=2001 |title=Soil particles reworking evidences by AMS ¹⁴C dating of charcoal |journal=Comptes Rendus de l'Académie des Sciences, Série IIA |volume=332 |issue=1 |pages=21–28 |url=https://www.researchgate.net/publication/238379602 |doi=10.1016/S1251-8050(00)01485-3 |bibcode=2001CRASE.332...21C |access-date=1 August 2021}} Fossil soil horizons from paleosols can be found within sedimentary rock sequences, allowing the study of past environments.{{cite journal |last=Retallack |first=Gregory J. |year=1991 |title=Untangling the effects of burial alteration and ancient soil formation |journal=Annual Review of Earth and Planetary Sciences |volume=19 |issue=1 |pages=183–206 |doi=10.1146/annurev.ea.19.050191.001151 |bibcode=1991AREPS..19..183R |url=https://www.researchgate.net/publication/234148901 |access-date=1 August 2021}}

The exposure of parent material to favourable conditions produces mineral soils that are marginally suitable for plant growth, as is the case in eroded soils.{{cite journal |last1=Bakker |first1=Martha M. |last2=Govers |first2=Gerard |last3=Jones |first3=Robert A. |last4=Rounsevell |first4=Mark D.A. |year=2007 |title=The effect of soil erosion on Europe's crop yields |journal=Ecosystems |volume=10 |issue=7 |pages=1209–1219 |doi=10.1007/s10021-007-9090-3 |bibcode=2007Ecosy..10.1209B |doi-access=free}} The growth of vegetation results in the production of organic residues which fall on the ground as litter for plant aerial parts (leaf litter) or are directly produced belowground for subterranean plant organs (root litter), and then release dissolved organic matter.{{cite journal |last1=Uselman |first1=Shauna M. |last2=Qualls |first2=Robert G. |last3=Lilienfein |first3=Juliane |year=2007 |title=Contribution of root vs. leaf litter to dissolved organic carbon leaching through soil |journal=Soil Science Society of America Journal |volume=71 |issue=5 |pages=1555–1563 |url=https://www.academia.edu/34475958 |doi=10.2136/sssaj2006.0386 |bibcode=2007SSASJ..71.1555U |access-date=8 August 2021}} The remaining surficial organic layer, called the O horizon, produces a more active soil due to the effect of the organisms that live within it. Organisms colonise and break down organic materials, making available nutrients upon which other plants and animals can live.{{cite journal |last1=Schulz |first1=Stefanie |last2=Brankatschk |first2=Robert |last3=Dümig |first3=Alexander |last4=Kögel-Knabner |first4=Ingrid|author4-link=Ingrid Kögel-Knabner |last5=Schloter |first5=Michae |last6=Zeyer |first6=Josef |year=2013 |title=The role of microorganisms at different stages of ecosystem development for soil formation |journal=Biogeosciences |volume=10 |issue=6 |pages=3983–3996 |doi=10.5194/bg-10-3983-2013 |bibcode=2013BGeo...10.3983S |doi-access=free}} After sufficient time, humus moves downward and is deposited in a distinctive organic-mineral surface layer called the A horizon, in which organic matter is mixed with mineral matter through the activity of burrowing animals, a process called pedoturbation. This natural process does not go to completion in the presence of conditions detrimental to soil life such as strong acidity, cold climate or pollution, stemming in the accumulation of undecomposed organic matter within a single organic horizon overlying the mineral soil{{cite journal |last1=Gillet |first1=Servane |last2=Ponge |first2=Jean-François |year=2002 |title=Humus forms and metal pollution in soil |journal=European Journal of Soil Science |volume=53 |issue=4 |pages=529–539 |url=https://www.academia.edu/45705588 |doi=10.1046/j.1365-2389.2002.00479.x |bibcode=2002EuJSS..53..529G |s2cid=94900982 |access-date=8 August 2021}} and in the juxtaposition of humified organic matter and mineral particles, without intimate mixing, in the underlying mineral horizons.{{cite journal |last1=Bardy |first1=Marion |last2=Fritsch |first2=Emmanuel |last3=Derenne |first3=Sylvie |last4=Allard |first4=Thierry |last5=do Nascimento |first5=Nadia Régina |last6=Bueno |first6=Guilherme |year=2008 |title=Micromorphology and spectroscopic characteristics of organic matter in waterlogged podzols of the upper Amazon basin |journal=Geoderma |volume=145 |issue=3 |pages=222–230 |doi=10.1016/j.geoderma.2008.03.008 |bibcode=2008Geode.145..222B |citeseerx=10.1.1.455.4179 }}

{{main|Soil classification}}

One of the first soil classification systems was developed by Russian scientist Vasily Dokuchaev around 1880.{{cite web |url=https://fr.scribd.com/doc/206859253/Russian-Chernozem |title=Russian Chernozem |last=Dokuchaev |first=Vasily Vasilyevich |publisher=Israel Program for Scientific Translations |location=Jerusalem, Israel |year=1967 |access-date=15 August 2021}} It was modified a number of times by American and European researchers and was developed into the system commonly used until the 1960s. It was based on the idea that soils have a particular morphology based on the materials and factors that form them. In the 1960s, a different classification system began to emerge which focused on soil morphology instead of parental materials and soil-forming factors. Since then, it has undergone further modifications. The World Reference Base for Soil Resources{{Cite web|url = https://www3.ls.tum.de/boku/?id=1419|title = World Reference Base for Soil Resources, 4th edition|author=IUSS Working Group WRB|year = 2022|publisher = IUSS, Vienna}} aims to establish an international reference base for soil classification.

Soil is used in agriculture, where it serves as the anchor and primary nutrient base for plants. The types of soil and available moisture determine the species of plants that can be cultivated. Agricultural soil science was the primeval domain of soil knowledge, long time before the advent of pedology in the 19th century. However, as demonstrated by aeroponics, aquaponics and hydroponics, soil material is not an absolute essential for agriculture, and soilless cropping systems have been claimed as the future of agriculture for an endless growing mankind.{{cite journal |last1=Sambo |first1=Paolo |last2=Nicoletto |first2=Carlo |last3=Giro |first3=Andrea |last4=Pii |first4=Youry |last5=Valentinuzzi |first5=Fabio |last6=Mimmo |first6=Tanja |last7=Lugli |first7=Paolo |last8=Orzes |first8=Guido |last9=Mazzetto |first9=Fabrizio |last10=Astolfi |first10=Stefania |last11=Terzano |first11=Roberto |last12=Cesco |first12=Stefano |year=2019 |title=Hydroponic solutions for soilless production systems: issues and opportunities in a smart agriculture perspective |journal=Frontiers in Plant Science |volume=10 |issue=123 |page=923 |doi=10.3389/fpls.2019.00923 |pmid=31396245 |pmc=6668597 |doi-access=free}}

Soil material is also a critical component in mining, construction and landscape development industries.{{cite book |title=Soils for landscape development: selection, specification and validation |last1=Leake |first1=Simon |last2=Haege |first2=Elke |publisher=CSIRO Publishing |location=Clayton, Victoria, Australia |year=2014 |isbn=978-0643109650}} Soil serves as a foundation for most construction projects. The movement of massive volumes of soil can be involved in surface mining, road building and dam construction. Earth sheltering is the architectural practice of using soil for external thermal mass against building walls. Many building materials are soil based. Loss of soil through urbanization is growing at a high rate in many areas and can be critical for the maintenance of subsistence agriculture.{{cite journal |last1=Pan |first1=Xian-Zhang |last2=Zhao |first2=Qi-Guo |year=2007 |title=Measurement of urbanization process and the paddy soil loss in Yixing city, China between 1949 and 2000 |journal=Catena |volume=69 |issue=1 |pages=65–73 |doi=10.1016/j.catena.2006.04.016 |bibcode=2007Caten..69...65P |url=http://www.cern.ac.cn/ftp/0301%20Measurement%20of%20urbanization%20process%20and%20the%20paddy%20soil%20loss%20in%20Yixing%20city,%20China%20between%201949%20and%202000).pdf |access-date=15 August 2021}}

Soil resources are critical to the environment, as well as to food and fibre production, producing 98.8% of food consumed by humans.{{cite journal |last1=Kopittke |first1=Peter M. |last2=Menzies |first2=Neal W. |last3=Wang |first3=Peng |last4=McKenna |first4=Brigid A. |last5=Lombi |first5=Enzo |year=2019 |title=Soil and the intensification of agriculture for global food security |journal=Environment International |volume=132 |pages=105078 |doi=10.1016/j.envint.2019.105078 |pmid=31400601 |issn=0160-4120 |doi-access=free|bibcode=2019EnInt.13205078K }} Soil provides minerals and water to plants according to several processes involved in plant nutrition. Soil absorbs rainwater and releases it later, thus preventing floods and drought, flood regulation being one of the major ecosystem services provided by soil.{{cite journal |last1=Stürck |first1=Julia |last2=Poortinga |first2=Ate |last3=Verburg |first3=Peter H. |year=2014 |title=Mapping ecosystem services: the supply and demand of flood regulation services in Europe |journal=Ecological Indicators |volume=38 |pages=198–211 |url=http://docs.gip-ecofor.org/public/Sturck_et_al_2014.pdf |doi=10.1016/j.ecolind.2013.11.010 |bibcode=2014EcInd..38..198S |access-date=15 August 2021 |archive-date=14 August 2021 |archive-url=https://web.archive.org/web/20210814121818/http://docs.gip-ecofor.org/public/Sturck_et_al_2014.pdf |url-status=dead }} Soil cleans water as it percolates through it.{{cite journal |last1=Van Cuyk |first1=Sheila |last2=Siegrist |first2=Robert |last3=Logan |first3=Andrew |last4=Masson |first4=Sarah |last5=Fischer |first5=Elizabeth |last6=Figueroa |first6=Linda |year=2001 |title=Hydraulic and purification behaviors and their interactions during wastewater treatment in soil infiltration systems |journal=Water Research |volume=35 |issue=4 |pages=953–964 |url=https://www.academia.edu/17525373 |doi=10.1016/S0043-1354(00)00349-3 |pmid=11235891 |bibcode=2001WatRe..35..953V |access-date=15 August 2021}} Soil is the habitat for many organisms: the major part of known and unknown biodiversity is in the soil, in the form of earthworms, woodlice, millipedes, centipedes, snails, slugs, mites, springtails, enchytraeids, nematodes, protists), bacteria, archaea, fungi and algae; and most organisms living above ground have part of them (plants) or spend part of their life cycle (insects) below-ground.{{cite book |title=European atlas of soil biodiversity |last1=Jeffery |first1=Simon |last2=Gardi |first2=Ciro |last3=Arwyn |first3=Jones |publisher=Publications Office of the European Union |location=Luxembourg, Luxembourg |year=2010 |isbn=978-92-79-15806-3 |doi=10.2788/94222 |url=https://op.europa.eu/en/publication-detail/-/publication/7161b2a1-f862-4c90-9100-557a62ecb908 |access-date=15 August 2021}} Above-ground and below-ground biodiversities are tightly interconnected,{{cite journal |last1=De Deyn |first1=Gerlinde B. |last2=Van der Putten |first2=Wim H. |year=2005 |title=Linking aboveground and belowground diversity |journal=Trends in Ecology and Evolution |volume=20 |issue=11 |pages=625–633 |url=https://www.researchgate.net/publication/7080980 |doi=10.1016/j.tree.2005.08.009 |pmid=16701446 |access-date=15 August 2021}} making soil protection of paramount importance for any restoration or conservation plan.

The biological component of soil is an extremely important carbon sink since about 57% of the biotic content is carbon. Even in deserts, cyanobacteria, lichens and mosses form biological soil crusts which capture and sequester a significant amount of carbon by photosynthesis. Poor farming and grazing methods have degraded soils and released much of this sequestered carbon to the atmosphere. Restoring the world's soils could offset the effect of increases in greenhouse gas emissions and slow global warming, while improving crop yields and reducing water needs.{{cite journal |last1= Hansen |first1=James |last2=Sato |first2=Makiko |last3=Kharecha |first3=Pushker |last4=Beerling |first4=David |last5=Berner |first5=Robert |last6=Masson-Delmotte |first6=Valerie |last7=Pagani |first7=Mark |last8=Raymo |first8=Maureen |last9=Royer |first9=Dana L. |last10=Zachos |first10=James C. |journal=Open Atmospheric Science Journal |year=2008 |volume=2 |pages=217–231 |title=Target atmospheric CO₂: where should humanity aim? |issue=1 |arxiv=0804.1126 |bibcode=2008OASJ....2..217H |doi= 10.2174/1874282300802010217 |doi-access=free|s2cid=14890013 |url=https://benthamopen.com/contents/pdf/TOASCJ/TOASCJ-2-217.pdf |access-date=22 August 2021}}{{cite journal |last=Lal |first=Rattan |date=11 June 2004 |title=Soil carbon sequestration impacts on global climate change and food security |journal=Science |volume=304 |issue=5677 |pages=1623–1627 |doi=10.1126/science.1097396 |pmid=15192216 |bibcode=2004Sci...304.1623L |s2cid=8574723 |url=http://www.tinread.usarb.md:8888/jspui/bitstream/123456789/1067/1/soil_carbon.pdf |access-date=22 August 2021 |archive-date=14 August 2021 |archive-url=https://web.archive.org/web/20210814145106/http://www.tinread.usarb.md:8888/jspui/bitstream/123456789/1067/1/soil_carbon.pdf |url-status=dead }}{{cite web |last=Blakeslee |first=Thomas |title=Greening deserts for carbon credits |date=24 February 2010 |access-date=22 August 2021 |publisher=Renewable Energy World |location=Orlando, Florida, USA |url=https://www.renewableenergyworld.com/om/greening-deserts-for-carbon-credits/#gref |url-status=live |archive-url=https://web.archive.org/web/20121101011735/http://www.renewableenergyworld.com/rea/news/article/2010/02/greening-deserts-for-carbon-credits |archive-date=1 November 2012}}

Waste management often has a soil component. Septic drain fields treat septic tank effluent using aerobic soil processes. Land application of waste water relies on soil biology to aerobically treat BOD. Alternatively, landfills use soil for daily cover, isolating waste deposits from the atmosphere and preventing unpleasant smells. Composting is now widely used to treat aerobically solid domestic waste and dried effluents of settling basins. Although compost is not soil, biological processes taking place during composting are similar to those occurring during decomposition and humification of soil organic matter.{{cite journal |last1=Mondini |first1=Claudio |last2=Contin |first2=Marco |last3=Leita |first3=Liviana |last4=De Nobili |first4=Maria |year=2002 |title=Response of microbial biomass to air-drying and rewetting in soils and compost |journal=Geoderma |volume=105 |issue=1–2 |pages=111–124 |url=https://www.academia.edu/5321925 |doi=10.1016/S0016-7061(01)00095-7 |bibcode=2002Geode.105..111M |access-date=22 August 2021}}

Organic soils, especially peat, serve as a significant fuel and horticultural resource. Peat soils are also commonly used for the sake of agriculture in Nordic countries, because peatland sites, when drained, provide fertile soils for food production.{{cite web |title=Peatlands and farming |date=6 July 2020 |access-date=22 August 2021 |publisher=National Farmers' Union of England and Wales |location=Stoneleigh, United Kingdom |url=https://www.countrysideonline.co.uk/food-and-farming/protecting-the-environment/peatlands-and-farming |archive-url=https://web.archive.org/web/20200514100050/https://www.countrysideonline.co.uk/food-and-farming/protecting-the-environment/peatlands-and-farming/ |url-status=dead |archive-date=14 May 2020 }} However, wide areas of peat production, such as rain-fed sphagnum bogs, also called blanket bogs or raised bogs, are now protected because of their patrimonial interest. As an example, Flow Country, covering 4,000 square kilometres of rolling expanse of blanket bogs in Scotland, is now candidate for being included in the World Heritage List. Under present-day global warming peat soils are thought to be involved in a self-reinforcing (positive feedback) process of increased emission of greenhouse gases (methane and carbon dioxide) and increased temperature,{{cite journal |last1=van Winden |first1=Julia F. |last2=Reichart |first2=Gert-Jan |last3=McNamara |first3=Niall P. |last4=Benthien |first4=Albert |last5=Sinninghe Damste |first5=Jaap S. |journal=PLoS ONE |year=2012 |volume=7 |issue=6 |pages=e39614 |title=Temperature-induced increase in methane release from peat bogs: a mesocosm experiment |doi=10.1371/journal.pone.0039614 |pmid=22768100 |pmc=3387254 |bibcode=2012PLoSO...739614V |doi-access=free}} a contention which is still under debate when replaced at field scale and including stimulated plant growth.{{cite journal |last1=Davidson |first1=Eric A. |last2=Janssens |first2=Ivan A. |year=2006 |title=Temperature sensitivity of soil carbon decomposition and feedbacks to climate change |journal=Nature |volume=440 |issue=7081 |pages=165–173 |doi=10.1038/nature04514 |pmid=16525463 |bibcode=2006Natur.440..165D |s2cid=4404915 |doi-access=free }}

Geophagy is the practice of eating soil-like substances. Both animals and humans occasionally consume soil for medicinal, recreational, or religious purposes.{{cite journal |last=Abrahams |first=Pter W. |year=1997 |title=Geophagy (soil consumption) and iron supplementation in Uganda |journal=Tropical Medicine and International Health |volume=2 |issue=7 |pages=617–623 |doi=10.1046/j.1365-3156.1997.d01-348.x |pmid=9270729 |s2cid=19647911 |doi-access=free}} It has been shown that some monkeys consume soil, together with their preferred food (tree foliage and fruits), in order to alleviate tannin toxicity.{{cite journal|last1=Setz |first1=Eleonore Zulnara Freire |last2=Enzweiler |first2=Jacinta |last3=Solferini |first3=Vera Nisaka |last4=Amêndola |first4=Monica Pimenta |last5=Berton |first5=Ronaldo Severiano |year=1999 |title=Geophagy in the golden-faced saki monkey (Pithecia pithecia chrysocephala) in the Central Amazon |journal=Journal of Zoology |volume=247 |issue=1 |pages=91–103 |doi=10.1111/j.1469-7998.1999.tb00196.x |url=https://www.academia.edu/26464333 |access-date=22 August 2021}}

Soils filter and purify water and affect its chemistry. Rain water and pooled water from ponds, lakes and rivers percolate through the soil horizons and the upper rock strata, thus becoming groundwater. Pests (viruses) and pollutants, such as persistent organic pollutants (chlorinated pesticides, polychlorinated biphenyls), oils (hydrocarbons), heavy metals (lead, zinc, cadmium), and excess nutrients (nitrates, sulfates, phosphates) are filtered out by the soil.{{cite journal |last1=Kohne |first1=John Maximilian |last2=Koehne |first2=Sigrid |last3=Simunek |first3=Jirka |date=2009 |title=A review of model applications for structured soils: a) Water flow and tracer transport |journal=Journal of Contaminant Hydrology |volume=104 |pages=4–35 |doi=10.1016/j.jconhyd.2008.10.002 |pmid=19012994 |issue=1–4 |bibcode=2009JCHyd.104....4K |url=http://www.pc-progress.com/documents/jirka/ko-ko_sim_2008_jcontamhydrol.pdf |url-status=live |archive-url=https://web.archive.org/web/20171107005433/http://www.soil.tu-bs.de/lehre/Master.Monitoring/2009/Daten/5_Literatur/A%20review%20of-Koehne-2009.pdf |archive-date=7 November 2017 |citeseerx=10.1.1.468.9149 |access-date=22 August 2021}} Soil organisms metabolise them or immobilise them in their biomass and necromass,{{Cite journal|last1=Diplock |first1=Elizabeth E. |last2=Mardlin |first2=Dave P. |last3=Killham |first3=Kenneth S. |last4=Paton |first4=Graeme Iain |year=2009 |title=Predicting bioremediation of hydrocarbons: laboratory to field scale |journal=Environmental Pollution |volume=157 |pages=1831–1840 |doi=10.1016/j.envpol.2009.01.022 |pmid=19232804 |issue=6 |bibcode=2009EPoll.157.1831D |url=https://coek.info/pdf-predicting-bioremediation-of-hydrocarbons-laboratory-to-field-scale-.html |access-date=22 August 2021}} thereby incorporating them into stable humus.{{cite journal |last1=Moeckel |first1=Claudia |last2=Nizzetto |first2=Luca |last3=Di Guardo |first3=Antonio |last4=Steinnes |first4=Eiliv |last5=Freppaz |first5=Michele |last6=Filippa |first6=Gianluca |last7=Camporini |first7=Paolo |last8=Benner |first8=Jessica |last9=Jones |first9=Kevin C. |date=2008 |title=Persistent organic pollutants in boreal and montane soil profiles: distribution, evidence of processes and implications for global cycling |journal=Environmental Science and Technology |volume=42 |pages=8374–8380 |doi=10.1021/es801703k |pmid=19068820 |issue=22 |bibcode=2008EnST...42.8374M |hdl=11383/8693 |url=https://www.academia.edu/15598352 |access-date=22 August 2021|hdl-access=free }} The physical integrity of soil is also a prerequisite for avoiding landslides in rugged landscapes.{{cite journal |last1=Rezaei |first1=Khalil |last2=Guest |first2=Bernard |last3=Friedrich |first3=Anke |last4=Fayazi |first4=Farajollah |last5=Nakhaei |first5=Mohamad |last6=Aghda |first6=Seyed Mahmoud Fatemi |last7=Beitollahi |first7=Ali |date=2009 |title=Soil and sediment quality and composition as factors in the distribution of damage at the December 26, 2003, Bam area earthquake in SE Iran (M (s)=6.6) |journal=Journal of Soils and Sediments |volume=9 |issue=1 |pages=23–32 |doi=10.1007/s11368-008-0046-9 |bibcode=2009JSoSe...9...23R |s2cid=129416733 |url=https://www.researchgate.net/publication/225752596 |access-date=22 August 2021}}

{{Main|Soil retrogression and degradation|Soil conservation}}

Land degradation is a human-induced or natural process which impairs the capacity of land to function.{{cite journal |last1=Johnson |first1=Dan L. |last2=Ambrose |first2=Stanley H. |last3=Bassett |first3=Thomas J. |last4=Bowen |first4=Merle L. |last5=Crummey |first5=Donald E. |last6=Isaacson |first6=John S. |last7=Johnson |first7=David N. |last8=Lamb |first8=Peter |last9=Saul |first9=Mahir |last10=Winter-Nelson |first10=Alex E. |year=1997 |title=Meanings of environmental terms |url=https://www.researchgate.net/publication/240784159 |journal=Journal of Environmental Quality |volume=26 |issue=3 |pages=581–589 |doi=10.2134/jeq1997.00472425002600030002x |bibcode=1997JEnvQ..26..581J |access-date=29 August 2021}} Soil degradation involves acidification, contamination, desertification, erosion or salination.{{cite book |last=Oldeman |first=L. Roel |year=1993 |chapter=Global extent of soil degradation |title=ISRIC Bi-Annual Report 1991–1992 |pages=19–36 |chapter-url=https://library.wur.nl/WebQuery/wurpubs/fulltext/299739 |publisher=International Soil Reference and Information Centre(ISRIC) |location=Wageningen, The Netherlands |access-date=29 August 2021}}

Soil acidification is beneficial in the case of alkaline soils, but it degrades land when it lowers crop productivity, soil biological activity and increases soil vulnerability to contamination and erosion. Soils are initially acid and remain such when their parent materials are low in basic cations (calcium, magnesium, potassium and sodium). On parent materials richer in weatherable minerals acidification occurs when basic cations are leached from the soil profile by rainfall or exported by the harvesting of forest or agricultural crops. Soil acidification is accelerated by the use of acid-forming nitrogenous fertilizers and by the effects of acid precipitation. Deforestation is another cause of soil acidification, mediated by increased leaching of soil nutrients in the absence of tree canopies.{{cite book |last1=Sumner |first1=Malcolm E. |last2=Noble |first2=Andrew D. |year=2003 |chapter=Soil acidification: the world story |title=Handbook of soil acidity |pages=1–28 |editor-last=Rengel |editor-first=Zdenko |chapter-url=https://pdf-drive.com/pdf/Zdenko20Rengel20-20Handbook20of20Soil20Acidity2028Books20in20Soils2C20Plants2C20and20the20Environment292028200329.pdf#page=16 |publisher=Marcel Dekker |location=New York, NY, USA |access-date=29 August 2021 |archive-date=14 August 2021 |archive-url=https://web.archive.org/web/20210814115102/https://pdf-drive.com/pdf/Zdenko20Rengel20-20Handbook20of20Soil20Acidity2028Books20in20Soils2C20Plants2C20and20the20Environment292028200329.pdf#page=16 |url-status=dead }}

Soil contamination at low levels is often within a soil's capacity to treat and assimilate waste material. Soil biota can treat waste by transforming it, mainly through microbial enzymatic activity.{{cite journal |last1=Karam |first1=Jean |last2=Nicell |first2=James A. |year=1997 |title=Potential applications of enzymes in waste treatment |url=https://www.researchgate.net/publication/30002097 |journal=Journal of Chemical Technology & Biotechnology |volume=69 |issue=2 |pages=141–153 |doi=10.1002/(SICI)1097-4660(199706)69:2<141::AID-JCTB694>3.0.CO;2-U |bibcode=1997JCTB...69..141K |access-date=5 September 2021}} Soil organic matter and soil minerals can adsorb the waste material and decrease its toxicity,{{cite journal |last1=Sheng |first1=Guangyao |last2=Johnston |first2=Cliff T. |last3=Teppen |first3=Brian J. |last4=Boyd |first4=Stephen A. |year=2001 |title=Potential contributions of smectite clays and organic matter to pesticide retention in soils |url=https://www.academia.edu/4875079 |journal=Journal of Agricultural and Food Chemistry |volume=49 |issue=6 |pages=2899–2907 |doi=10.1021/jf001485d |pmid=11409985 |bibcode=2001JAFC...49.2899S |access-date=5 September 2021}} although when in colloidal form they may transport the adsorbed contaminants to subsurface environments.{{cite journal |last1=Sprague |first1=Lori A. |last2=Herman |first2=Janet S. |last3=Hornberger |first3=George M. |last4=Mills |first4=Aaron L. |year=2000 |title=Atrazine adsorption and colloid-facilitated transport through the unsaturated zone |url=https://lmecol.evsc.virginia.edu/pubs/73-Sprague_JEQ2000.pdf |journal=Journal of Environmental Quality |volume=29 |issue=5 |pages=1632–1641 |doi=10.2134/jeq2000.00472425002900050034x |bibcode=2000JEnvQ..29.1632S |access-date=5 September 2021 |archive-date=14 August 2021 |archive-url=https://web.archive.org/web/20210814121821/https://lmecol.evsc.virginia.edu/pubs/73-Sprague_JEQ2000.pdf |url-status=dead }} Many waste treatment processes rely on this natural bioremediation capacity. Exceeding treatment capacity can damage soil biota and limit soil function. Derelict soils occur where industrial contamination or other development activity damages the soil to such a degree that the land cannot be used safely or productively. Remediation of derelict soil uses principles of geology, physics, chemistry and biology to degrade, attenuate, isolate or remove soil contaminants to restore soil functions and values. Techniques include leaching, air sparging, soil conditioners, phytoremediation, bioremediation and Monitored Natural Attenuation. An example of diffuse pollution with contaminants is copper accumulation in vineyards and orchards to which fungicides are repeatedly applied, even in organic farming.{{Cite journal |last1=Ballabio |first1=Cristiano |last2=Panagos |first2=Panos |last3=Lugato |first3=Emanuele |last4=Huang |first4=Jen-How |last5=Orgiazzi |first5=Alberto |last6=Jones |first6=Arwyn |last7=Fernández-Ugalde |first7=Oihane |last8=Borrelli |first8=Pasquale |last9=Montanarella |first9=Luca |date=15 September 2018 |title=Copper distribution in European topsoils: an assessment based on LUCAS soil survey |journal=Science of the Total Environment |volume=636 |pages=282–298 |doi=10.1016/j.scitotenv.2018.04.268 |pmid=29709848 |issn=0048-9697 |bibcode=2018ScTEn.636..282B |doi-access=free}}

Microfibres from synthetic textiles are another type of plastic soil contamination, 100% of agricultural soil samples from southwestern China contained plastic particles, 92% of which were microfibres. Sources of microfibres likely included string or twine, as well as irrigation water in which clothes had been washed.{{Cite web |last=Environment |first=U. N. |date=2021-10-21 |title=Drowning in Plastics – Marine Litter and Plastic Waste Vital Graphics |url=http://www.unep.org/resources/report/drowning-plastics-marine-litter-and-plastic-waste-vital-graphics |access-date=2022-03-23 |website=UNEP - UN Environment Programme |language=en}}

The application of biosolids from sewage sludge and compost can introduce microplastics to soils. This adds to the burden of microplastics from other sources (e.g. the atmosphere). Approximately half the sewage sludge in Europe and North America is applied to agricultural land. In Europe it has been estimated that for every million inhabitants 113 to 770 tonnes of microplastics are added to agricultural soils each year.

File:Soil erosion, Southfield - geograph.org.uk - 367917.jpg

Desertification, an environmental process of ecosystem degradation in arid and semi-arid regions, is often caused by badly adapted human activities such as overgrazing or excess harvesting of firewood. It is a common misconception that drought causes desertification.{{Cite journal |last=Le Houérou |first=Henry N. |year=1996 |title=Climate change, drought and desertification |journal=Journal of Arid Environments |volume=34 |issue=2 |pages=133–185 |doi=10.1006/jare.1996.0099 |bibcode=1996JArEn..34..133L |url=http://www7.nau.edu/mpcer/direnet/publications/publications_l/files/LeHouerou_1996.pdf |access-date=5 September 2021}} Droughts are common in arid and semiarid lands. Well-managed lands can recover from drought when the rains return. Soil management tools include maintaining soil nutrient and organic matter levels, reduced tillage and increased cover.{{Cite journal |last1=Lyu |first1=Yanli |last2=Shi |first2=Peijun |last3=Han |first3=Guoyi |last4=Liu |first4=Lianyou |last5=Guo |first5=Lanlan |last6=Hu |first6=Xia |last7=Zhang |first7=Guoming |year=2020 |title=Desertification control practices in China |journal=Sustainability |volume=12 |issue=8 |pages=3258 |doi=10.3390/su12083258 |issn=2071-1050 |doi-access=free|bibcode=2020Sust...12.3258L }} These practices help to control erosion and maintain productivity during periods when moisture is available. Continued land abuse during droughts, however, increases land degradation. Increased population and livestock pressure on marginal lands accelerates desertification.{{Cite journal |last1=Kéfi |first1=Sonia |last2=Rietkerk |first2=Max |last3=Alados |first3=Concepción L. |last4=Pueyo |first4=Yolanda |last5=Papanastasis |first5=Vasilios P. |last6=El Aich |first6=Ahmed |last7=de Ruiter |first7=Peter C. |year=2007 |title=Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems |journal=Nature |volume=449 |issue=7159 |pages=213–217 |doi=10.1038/nature06111 |pmid=17851524 |bibcode=2007Natur.449..213K |hdl=1874/25682 |s2cid=4411922 |url=https://www.researchgate.net/publication/232801317 |access-date=5 September 2021}} It is now questioned whether present-day climate warming will favour or disfavour desertification, with contradictory reports about predicted rainfall trends associated with increased temperature, and strong discrepancies among regions, even in the same country.{{Cite journal |last1=Wang |first1=Xunming |last2=Yang |first2=Yi |last3=Dong |first3=Zhibao |last4=Zhang |first4=Caixia |year=2009 |title=Responses of dune activity and desertification in China to global warming in the twenty-first century |journal=Global and Planetary Change |volume=67 |issue=3–4 |pages=167–185 |doi=10.1016/j.gloplacha.2009.02.004 |bibcode=2009GPC....67..167W |url=https://www.researchgate.net/publication/229103975 |access-date=5 September 2021}}

File:Riparian buffer on Bear Creek in Story County, Iowa.JPG

Erosion of soil is caused by water, wind, ice, and movement in response to gravity. More than one kind of erosion can occur simultaneously. Erosion is distinguished from weathering, since erosion also transports eroded soil away from its place of origin (soil in transit may be described as sediment). Erosion is an intrinsic natural process, but in many places it is greatly increased by human activity, especially unsuitable land use practices.{{Cite journal |last1=Yang |first1=Dawen |last2=Kanae |first2=Shinjiro |last3=Oki |first3=Taikan |last4=Koike |first4=Toshio |last5=Musiake |first5=Katumi |year=2003 |title=Global potential soil erosion with reference to land use and climate changes |journal=Hydrological Processes |volume=17 |issue=14 |pages=2913–28 |doi=10.1002/hyp.1441 |bibcode=2003HyPr...17.2913Y |s2cid=129355387 |url=https://www.oieau.org/eaudoc/system/files/documents/38/191115/191115_doc.pdf |access-date=5 September 2021 |archive-date=18 August 2021 |archive-url=https://web.archive.org/web/20210818043117/https://www.oieau.org/eaudoc/system/files/documents/38/191115/191115_doc.pdf |url-status=dead }} These include agricultural activities which leave the soil bare during times of heavy rain or strong winds, overgrazing, deforestation, and improper construction activity. Improved management can limit erosion. Soil conservation techniques which are employed include changes of land use (such as replacing erosion-prone crops with grass or other soil-binding plants), changes to the timing or type of agricultural operations, terrace building, use of erosion-suppressing cover materials (including cover crops and other plants), limiting disturbance during construction, and avoiding construction during erosion-prone periods and in erosion-prone places such as steep slopes.{{Cite journal |last1=Sheng |first1=Jian-an |last2=Liao |first2=An-zhong |year=1997 |title=Erosion control in South China |journal=Catena |issn=0341-8162 |volume=29 |issue=2 |pages=211–221 |doi=10.1016/S0341-8162(96)00057-4 |bibcode=1997Caten..29..211S |url=https://coek.info/pdf-erosion-control-in-south-china-.html |access-date=5 September 2021}} Historically, one of the best examples of large-scale soil erosion due to unsuitable land-use practices is wind erosion (the so-called dust bowl) which ruined American and Canadian prairies during the 1930s, when immigrant farmers, encouraged by the federal government of both countries, settled and converted the original shortgrass prairie to agricultural crops and cattle ranching.

A serious and long-running water erosion problem occurs in China, on the middle reaches of the Yellow River and the upper reaches of the Yangtze River. From the Yellow River, over 1.6 billion tons of sediment flow each year into the ocean. The sediment originates primarily from water erosion (gully erosion) in the Loess Plateau region of northwest China.{{Cite journal |last1=Ran |first1=Lishan |last2=Lu |first2=Xi Xi |last3=Xin |first3=Zhongbao |year=2014 |title=Erosion-induced massive organic carbon burial and carbon emission in the Yellow River basin, China |journal=Biogeosciences |volume=11 |issue=4 |pages=945–959 |doi=10.5194/bg-11-945-2014 |bibcode=2014BGeo...11..945R |url=https://bg.copernicus.org/articles/11/945/2014/bg-11-945-2014.pdf |access-date=5 September 2021 |hdl=10722/228184 |hdl-access=free |doi-access=free }}

Soil piping is a particular form of soil erosion that occurs below the soil surface.{{Cite journal |last1=Verachtert |first1=Els |last2=Van den Eeckhaut |first2=Miet |last3=Poesen |first3=Jean |last4=Deckers |first4=Jozef |year=2010 |title=Factors controlling the spatial distribution of soil piping erosion on loess-derived soils: a case study from central Belgium |journal=Geomorphology |volume=118 |issue=3 |pages=339–348 |doi=10.1016/j.geomorph.2010.02.001 |bibcode=2010Geomo.118..339V |url=https://lirias.kuleuven.be/retrieve/109942 |access-date=5 September 2021}} It causes levee and dam failure, as well as sink hole formation. Turbulent flow removes soil starting at the mouth of the seep flow and the subsoil erosion advances up-gradient.{{Cite journal |last=Jones |first=Anthony |title=Soil piping and stream channel initiation |journal=Water Resources Research |volume=7 |issue=3 |pages=602–610 |year=1976 |doi=10.1029/WR007i003p00602 |bibcode=1971WRR.....7..602J |url=https://booksc.eu/book/20668631/3ac27a |access-date=5 September 2021 |archive-date=5 September 2021 |archive-url=https://web.archive.org/web/20210905084319/https://booksc.eu/book/20668631/3ac27a |url-status=dead }} The term sand boil is used to describe the appearance of the discharging end of an active soil pipe.{{cite web|last=Dooley |first=Alan |title=Sandboils 101: Corps has experience dealing with common flood danger |website=Engineer Update |publisher=US Army Corps of Engineers |date=June 2006 |url=http://www.hq.usace.army.mil/cepa/pubs/jun06/story8.htm |archive-url=https://web.archive.org/web/20080418185527/http://www.hq.usace.army.mil/cepa/pubs/jun06/story8.htm |archive-date=18 April 2008 |url-status=dead}}

Soil salination is the accumulation of free salts to such an extent that it leads to degradation of the agricultural value of soils and vegetation. Consequences include corrosion damage, reduced plant growth, erosion due to loss of plant cover and soil structure, and water quality problems due to sedimentation. Salination occurs due to a combination of natural and human-caused processes. Arid conditions favour salt accumulation. This is especially apparent when soil parent material is saline. Irrigation of arid lands is especially problematic.{{cite web |last=Oosterbaan |first=Roland J. |title=Effectiveness and social/environmental impacts of irrigation projects: a critical review |series=Annual Reports of the International Institute for Land Reclamation and Improvement (ILRI) |year=1988 |pages=18–34 |location=Wageningen, The Netherlands |url=http://www.waterlog.info/pdf/irreff.pdf |url-status=live |archive-url=https://web.archive.org/web/20090219070320/http://waterlog.info/pdf/irreff.pdf |archive-date=19 February 2009 |df=dmy-all |access-date=5 September 2021}} All irrigation water has some level of salinity. Irrigation, especially when it involves leakage from canals and overirrigation in the field, often raises the underlying water table. Rapid salination occurs when the land surface is within the capillary fringe of saline groundwater. Soil salinity control involves watertable control and flushing with higher levels of applied water in combination with tile drainage or another form of subsurface drainage.{{Cite book |title=Drainage manual: a guide to integrating plant, soil, and water relationships for drainage of irrigated lands |year=1993 |publisher=United States Department of the Interior, Bureau of Reclamation |location=Washington, D.C. |url=https://www.usbr.gov/tsc/techreferences/mands/mands-pdfs/DrainMan.pdf |isbn=978-0-16-061623-5 |access-date=5 September 2021}}{{cite web |last=Oosterbaan |first=Roland J. |url=http://www.waterlog.info |title=Waterlogging, soil salinity, field irrigation, plant growth, subsurface drainage, groundwater modelling, surface runoff, land reclamation, and other crop production and water management aspects |access-date=5 September 2021 |url-status=live |archive-url=https://web.archive.org/web/20100816225219/http://www.waterlog.info/ |archive-date=16 August 2010}}

{{Main|Soil regeneration}}

Soils which contain high levels of particular clays with high swelling properties, such as smectites, are often very fertile. For example, the smectite-rich paddy soils of Thailand's Central Plains are among the most productive in the world. However, the overuse of mineral nitrogen fertilizers and pesticides in irrigated intensive rice production has endangered these soils, forcing farmers to implement integrated practices based on Cost Reduction Operating Principles.{{cite journal |last1=Stuart |first1=Alexander M. |last2=Pame |first2=Anny Ruth P. |last3=Vithoonjit |first3=Duangporn |last4=Viriyangkura |first4=Ladda |last5=Pithuncharurnlap |first5=Julmanee |last6=Meesang |first6=Nisa |last7=Suksiri |first7=Prarthana |last8=Singleton |first8=Grant R. |last9=Lampayan |first9=Rubenito M. |year=2018 |title=The application of best management practices increases the profitability and sustainability of rice farming in the central plains of Thailand |url=https://www.researchgate.net/publication/314091782 |journal=Field Crops Research |volume=220 |pages=78–87 |doi=10.1016/j.fcr.2017.02.005 |bibcode=2018FCrRe.220...78S |access-date=12 September 2021}}

Many farmers in tropical areas, however, struggle to retain organic matter and clay in the soils they work. In recent years, for example, productivity has declined and soil erosion has increased in the low-clay soils of northern Thailand, following the abandonment of shifting cultivation for a more permanent land use.{{cite journal |last1=Turkelboom |first1=Francis |last2=Poesen |first2=Jean |last3=Ohler |first3=Ilse |last4=Van Keer |first4=Koen |last5=Ongprasert |first5=Somchai |last6=Vlassak |first6=Karel |year=1997 |title=Assessment of tillage erosion rates on steep slopes in northern Thailand |url=https://www.academia.edu/17993140 |journal=Catena |volume=29 |issue=1 |pages=29–44 |doi=10.1016/S0341-8162(96)00063-X |bibcode=1997Caten..29...29T |access-date=12 September 2021}} Farmers initially responded by adding organic matter and clay from termite mound material, but this was unsustainable in the long-term because of rarefaction of termite mounds. Scientists experimented with adding bentonite, one of the smectite family of clays, to the soil. In field trials, conducted by scientists from the International Water Management Institute (IWMI) in cooperation with Khon Kaen University and local farmers, this had the effect of helping retain water and nutrients. Supplementing the farmer's usual practice with a single application of {{convert|200|kg/rai|kg/hectare lb/acre|lk=in}} of bentonite resulted in an average yield increase of 73%.{{cite journal |last1=Saleth |first1=Rathinasamy Maria |last2=Inocencio |first2=Arlene |last3=Noble |first3=Andrew |last4=Ruaysoongnern |first4=Sawaeng |year=2009 |title=Economic gains of improving soil fertility and water holding capacity with clay application: the impact of soil remediation research in Northeast Thailand |url=https://ageconsearch.umn.edu/record/53064/files/RR130.pdf |journal=Journal of Development Effectiveness |volume=1 |issue=3 |pages=336–352 |doi=10.1080/19439340903105022 |s2cid=18049595 |access-date=12 September 2021}} Other studies showed that applying bentonite to degraded sandy soils reduced the risk of crop failure during drought years.{{cite journal |last1=Semalulu |first1=Onesmus |last2=Magunda |first2=Matthias |last3=Mubiru |first3=Drake N. |year=2015 |title=Amelioration of sandy soils in drought stricken areas through use of Ca-bentonite |url=https://www.ajol.info/index.php/ujas/article/download/141752/131487 |journal=Uganda Journal of Agricultural Sciences |volume=16 |issue=2 |pages=195–205 |doi=10.4314/ujas.v16i2.5 |access-date=12 September 2021 |doi-access=free}}

In 2008, three years after the initial trials, IWMI scientists conducted a survey among 250 farmers in northeast Thailand, half of whom had applied bentonite to their fields. The average improvement for those using the clay addition was 18% higher than for non-clay users. Using the clay had enabled some farmers to switch to growing vegetables, which need more fertile soil. This helped to increase their income. The researchers estimated that 200 farmers in northeast Thailand and 400 in Cambodia had adopted the use of clays, and that a further 20,000 farmers were introduced to the new technique.{{cite journal |year=2010 |url=http://www.iwmi.cgiar.org/Publications/Success_Stories/PDF/2010/Issue%202%20-%20Improving_soils_and_boosting_yields_in_Thailand.pdf |title=Improving soils and boosting yields in Thailand |doi=10.5337/2011.0031 |journal=Success Stories |issue=2 |url-status=live |archive-url=https://web.archive.org/web/20120607030912/http://www.iwmi.cgiar.org/Publications/Success_Stories/PDF/2010/Issue%202%20-%20Improving_soils_and_boosting_yields_in_Thailand.pdf |archive-date=7 June 2012 |author=International Water Management Institute |doi-access=free |access-date=12 September 2021}}

If the soil is too high in clay or salts (e.g. saline sodic soil), adding gypsum, washed river sand and organic matter (e.g.municipal solid waste) will balance the composition.{{cite journal |last1=Prapagar |first1=Komathy |last2=Indraratne |first2=Srimathie P. |last3=Premanandharajah |first3=Punitha |year=2012 |title=Effect of soil amendments on reclamation of saline-sodic soil |url=https://www.researchgate.net/publication/267202667 |journal=Tropical Agricultural Research |volume=23 |issue=2 |pages=168–176 |doi=10.4038/tar.v23i2.4648 |access-date=12 September 2021 |doi-access=free}}

Adding organic matter, like ramial chipped wood or compost, to soil which is depleted in nutrients and too high in sand will boost its quality and improve production.{{cite web |last1=Lemieux |first1=Gilles |last2=Germain |first2=Diane |title=Ramial chipped wood: the clue to a sustainable fertile soil |publisher=Université Laval, Département des Sciences du Bois et de la Forêt, Québec, Canada |date=December 2000 |url=https://www.healthy-vegetable-gardening.com/support-files/rcw-the-clue-to-a-sustainable-fertile-soil.pdf |access-date=12 September 2021 |archive-date=28 September 2021 |archive-url=https://web.archive.org/web/20210928080056/https://www.healthy-vegetable-gardening.com/support-files/rcw-the-clue-to-a-sustainable-fertile-soil.pdf |url-status=dead }}{{cite journal |last1=Arthur |first1=Emmanuel |last2=Cornelis |first2=Wim |last3=Razzaghi |first3=Fatemeh |year=2012 |title=Compost amendment of sandy soil affects soil properties and greenhouse tomato productivity |url=https://www.academia.edu/31660161 |journal=Compost Science and Utilization |volume=20 |issue=4 |pages=215–221 |doi=10.1080/1065657X.2012.10737051 |bibcode=2012CScUt..20..215A |s2cid=96896374 |access-date=12 September 2021}}

Special mention must be made of the use of charcoal, and more generally biochar to improve nutrient-poor tropical soils, a process based on the higher fertility of anthropogenic pre-Columbian Amazonian Dark Earths, also called Terra Preta de Índio, due to interesting physical and chemical properties of soil black carbon as a source of stable humus.{{cite journal |last1=Glaser |first1=Bruno |last2=Haumaier |first2=Ludwig |last3=Guggenberger |first3=Georg |last4=Zech |first4=Wolfgang |year=2001 |title=The 'Terra Preta' phenomenon: a model for sustainable agriculture in the humid tropics |url=https://www.researchgate.net/publication/12032464 |journal=Naturwissenschaften |volume=88 |issue=1 |pages=37–41 |doi=10.1007/s001140000193 |pmid=11302125 |bibcode=2001NW.....88...37G |s2cid=26608101 |access-date=12 September 2021}} However, the uncontrolled application of charred waste products of all kinds may endanger soil life and human health.{{cite journal |last1=Kavitha |first1=Beluri |last2=Pullagurala Venkata Laxma |first2=Reddy |last3=Kim |first3=Bojeong |last4=Lee |first4=Sang Soo |last5=Pandey |first5=Sudhir Kumar |last6=Kim |first6=Ki-Hyun |year=2018 |title=Benefits and limitations of biochar amendment in agricultural soils: a review |url=https://booksc.eu/book/72239607/440436 |journal=Journal of Environmental Management |volume=227 |pages=146–154 |doi=10.1016/j.jenvman.2018.08.082 |pmid=30176434 |bibcode=2018JEnvM.227..146K |s2cid=52168678 |access-date=12 September 2021 |archive-date=12 September 2021 |archive-url=https://web.archive.org/web/20210912081933/https://booksc.eu/book/72239607/440436 |url-status=dead }}

The history of the study of soil is intimately tied to humans' urgent need to provide food for themselves and forage for their animals. Throughout history, civilizations have prospered or declined as a function of the availability and productivity of their soils.{{cite book |last=Hillel |first=Daniel |year=1992 |title=Out of the Earth: civilization and the life of the soil |publisher=University of California Press |location=Berkeley, California |isbn=978-0-520-08080-5}}

{{Main|Soil fertility}}

The Greek historian Xenophon (450–355 BCE) was the first to expound upon the merits of green-manuring crops: 'But then whatever weeds are upon the ground, being turned into earth, enrich the soil as much as dung.'{{sfn|Donahue|Miller|Shickluna|1977|p=4}}

Columella's Of husbandry, {{circa|60 CE}}, advocated the use of lime and that clover and alfalfa (green manure) should be turned under,{{cite book |last=Columella |first=Lucius Junius Moderatus |year=1745 |title=Of husbandry, in twelve books, and his book concerning trees, with several illustrations from Pliny, Cato, Varro, Palladius, and other antient and modern authors, translated into English |publisher=Andrew Millar |location=London, United Kingdom |url=https://catalog.hathitrust.org/Record/005783003 |access-date=19 September 2021}} and was used by 15 generations (450 years) under the Roman Empire until its collapse.{{sfn|Donahue|Miller|Shickluna|1977|p=4}}{{sfn|Kellogg|1957|p=1}} From the fall of Rome to the French Revolution, knowledge of soil and agriculture was passed on from parent to child and as a result, crop yields were low. During the European Middle Ages, Yahya Ibn al-'Awwam's handbook,{{cite book |language=fr |last=Ibn al-'Awwam |year=1864 |title=Le livre de l'agriculture, traduit de l'arabe par Jean Jacques Clément-Mullet |series=Filāḥah.French. |publisher=Librairie A. Franck |location=Paris, France |url=https://catalog.hathitrust.org/Record/009953450 |access-date=19 September 2021}} with its emphasis on irrigation, guided the people of North Africa, Spain and the Middle East; a translation of this work was finally carried to the southwest of the United States when under Spanish influence.{{cite book |last=Jelinek |first=Lawrence J. |year=1982 |title=Harvest empire: a history of California agriculture |publisher=Boyd and Fraser |location=San Francisco, California |isbn=978-0-87835-131-2}} Olivier de Serres, considered the father of French agronomy, was the first to suggest the abandonment of fallowing and its replacement by hay meadows within crop rotations. He also highlighted the importance of soil (the French terroir) in the management of vineyards. His famous book {{Lang|fr|Le Théâtre d'Agriculture et mesnage des champs}}{{cite book |language=fr |last=de Serres |first=Olivier |year=1600 |title=Le Théâtre d'Agriculture et mesnage des champs |publisher=Jamet Métayer |location=Paris, France |url=https://gallica.bnf.fr/ark:/12148/bpt6k738381/f1.image |access-date=19 September 2021}} contributed to the rise of modern, sustainable agriculture and to the collapse of old agricultural practices such as soil amendment for crops by the lifting of forest litter and assarting, which ruined the soils of western Europe during the Middle Ages and even later on according to regions.{{cite journal |last1=Virto |first1=Iñigo |last2=Imaz |first2=María José |last3=Fernández-Ugalde |first3=Oihane |last4=Gartzia-Bengoetxea |first4=Nahia |last5=Enrique |first5=Alberto |last6=Bescansa |first6=Paloma |journal=Sustainability |volume=7 |issue=1 |title=Soil degradation and soil quality in western Europe: current situation and future perspectives |year=2015 |pages=313–365 |doi=10.3390/su7010313 |doi-access=free}}

Experiments into what made plants grow first led to the idea that the ash left behind when plant matter was burned was the essential element but overlooked the role of nitrogen, which is not left on the ground after combustion, a belief which prevailed until the 19th century.{{cite journal |last1=Van der Ploeg |first1=Rienk R. |last2=Schweigert |first2=Peter |last3=Bachmann |first3=Joerg |journal=Scientific World Journal |volume=1 |issue=S2 |title=Use and misuse of nitrogen in agriculture: the German story |year=2001 |pages=737–744 |doi=10.1100/tsw.2001.263 |pmid=12805882 |pmc=6084271 |doi-access=free}} In about 1635, the Flemish chemist Jan Baptist van Helmont thought he had proved water to be the essential element from his famous five years' experiment with a willow tree grown with only the addition of rainwater. His conclusion came from the fact that the increase in the plant's weight had apparently been produced only by the addition of water, with no reduction in the soil's weight.{{cite web |url=https://www.bbc.co.uk/bitesize/clips/zpgb4wx |title=Van Helmont's experiments on plant growth |website=BBC World Service |access-date=19 September 2021}}{{sfn|Kellogg|1957|p=3}} John Woodward ({{abbr|d.|died}} 1728) experimented with various types of water ranging from clean to muddy and found muddy water the best, and so he concluded that earthy matter was the essential element. Others concluded it was humus in the soil that passed some essence to the growing plant. Still others held that the vital growth principal was something passed from dead plants or animals to the new plants. At the start of the 18th century, Jethro Tull demonstrated that it was beneficial to cultivate (stir) the soil, but his opinion that the stirring made the fine parts of soil available for plant absorption was erroneous.{{sfn|Kellogg|1957|p=2}}

As chemistry developed, it was applied to the investigation of soil fertility. The French chemist Antoine Lavoisier showed in about 1778 that plants and animals must combust oxygen internally to live. He was able to deduce that most of the {{convert|165|lb|adj=on}} weight of van Helmont's willow tree derived from air.{{cite journal |language=fr |last=de Lavoisier |first=Antoine-Laurent |journal=Mémoires de l'Académie Royale des Sciences |title=Mémoire sur la combustion en général |year=1777 |url=http://www.academie-sciences.fr/pdf/dossiers/Franklin/Franklin_pdf/Mem1777_p592.pdf |access-date=19 September 2021}} It was the French agriculturalist Jean-Baptiste Boussingault who by means of experimentation obtained evidence showing that the main sources of carbon, hydrogen and oxygen for plants were air and water, while nitrogen was taken from soil.{{cite book |language=fr |last=Boussingault |first=Jean-Baptiste |title=Agronomie, chimie agricole et physiologie, volumes 1–5 |year=1860–1874 |publisher=Mallet-Bachelier |location=Paris, France |url=https://archive.org/details/8TSUP364_1 |access-date=19 September 2021}} Justus von Liebig in his book Organic chemistry in its applications to agriculture and physiology (published 1840), asserted that the chemicals in plants must have come from the soil and air and that to maintain soil fertility, the used minerals must be replaced.{{cite book |last=von Liebig |first=Justus |title=Organic chemistry in its applications to agriculture and physiology |year=1840 |publisher=Taylor and Walton |location=London |url=https://archive.org/details/organicchemistry00liebrich |access-date=19 September 2021}} Liebig nevertheless believed the nitrogen was supplied from the air. The enrichment of soil with guano by the Incas was rediscovered in 1802, by Alexander von Humboldt. This led to its mining and that of Chilean nitrate and to its application to soil in the United States and Europe after 1840.{{cite journal |last=Way |first=J. Thomas |journal=Journal of the Royal Agricultural Society of England |title=On the composition and money value of the different varieties of guano |year=1849 |volume=10 |pages=196–230 |url=https://www.biodiversitylibrary.org/item/37078#page/220/mode/1up |access-date=19 September 2021}}

The work of Liebig was a revolution for agriculture, and so other investigators started experimentation based on it. In England John Bennet Lawes and Joseph Henry Gilbert worked in the Rothamsted Experimental Station, founded by the former, and {{Not a typo|(re)discovered}} that plants took nitrogen from the soil, and that salts needed to be in an available state to be absorbed by plants. Their investigations also produced the superphosphate, consisting in the acid treatment of phosphate rock.{{sfn|Kellogg|1957|p=4}} This led to the invention and use of salts of potassium (K) and nitrogen (N) as fertilizers. Ammonia generated by the production of coke was recovered and used as fertiliser.{{cite web |last=Tandon |first=Hari L.S. |url=http://www.tandontech.net/fertilisers.html |title=A short history of fertilisers |website=Fertiliser Development and Consultation Organisation |access-date=17 December 2017 |archive-url=https://web.archive.org/web/20170123214241/http://www.tandontech.net/fertilisers.html |archive-date=23 January 2017 |url-status=dead}} Finally, the chemical basis of nutrients delivered to the soil in manure was understood and in the mid-19th century chemical fertilisers were applied. However, the dynamic interaction of soil and its life forms was still not understood.

In 1856, J. Thomas Way discovered that ammonia contained in fertilisers was transformed into nitrates,{{cite journal |last=Way |first=J. Thomas |journal=Journal of the Royal Agricultural Society of England |title=On the power of soils to absorb manure |year=1852 |volume=13 |pages=123–143 |url=https://biodiversitylibrary.org/page/45583402 |access-date=19 September 2021}} and twenty years later Robert Warington proved that this transformation was done by living organisms.{{cite book |last=Warington |first=Robert |title=Note on the appearance of nitrous acid during the evaporation of water: a report of experiments made in the Rothamsted laboratory |url=https://books.google.com/books?id=NlISAQAAMAAJ |year=1878 |publisher=Harrison and Sons |location=London, United Kingdom |access-date=19 September 2021}} In 1890 Sergei Winogradsky announced he had found the bacteria responsible for this transformation.{{cite journal |last=Winogradsky |first=Sergei |journal=Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences |title=Sur les organismes de la nitrification |language=fr |trans-title=On the organisms of nitrification |year=1890 |volume=110 |issue=1 |pages=1013–1016 |url=https://gallica.bnf.fr/ark:/12148/bpt6k30663/f1087?lang=EN |access-date=19 September 2021}}

It was known that certain legumes could take up nitrogen from the air and fix it to the soil but it took the development of bacteriology towards the end of the 19th century to lead to an understanding of the role played in nitrogen fixation by bacteria. The symbiosis of bacteria and leguminous roots, and the fixation of nitrogen by the bacteria, were simultaneously discovered by the German agronomist Hermann Hellriegel and the Dutch microbiologist Martinus Beijerinck.{{sfn|Kellogg|1957|p=4}}

Crop rotation, mechanisation, chemical and natural fertilisers led to a doubling of wheat yields in western Europe between 1800 and 1900.{{sfn|Kellogg|1957|pp=1–4}}

{{See also|Soil formation}}

Scientists who studied soil in connection with agricultural practices considered it mainly a static substrate. However, the soil is the result of evolution from more ancient geological materials under the action of biotic and abiotic processes. After studies of soil improvement commenced, other researchers began to study soil genesis and, as a result, soil types and classifications.

In 1860, while in Mississippi, Eugene W. Hilgard (1833–1916) studied the relationship between rock material, climate, vegetation, and the type of soils that were developed. He realised that the soils were dynamic and considered the classification of soil types.{{cite book |last=Hilgard |first=Eugene W. |title=Soils: their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions |year=1907 |publisher=The Macmillan Company |location=London, United Kingdom |url=https://www.biodiversitylibrary.org/bibliography/24461 |access-date=19 September 2021}} (See also at [https://www.gutenberg.org/ebooks/73975 Project Gutenberg]). His work was not continued. At about the same time, Friedrich Albert Fallou described soil profiles and related soil characteristics to their formation as part of his professional work evaluating forest and farmland for the principality of Saxony. His 1857 book, {{Lang|de|Anfangsgründe der Bodenkunde}} (First Principles of soil science), established modern soil science.{{cite book |language=de |last=Fallou |first=Friedrich Albert |title=Anfangsgründe der Bodenkunde |year= 1857 |publisher=G. Schönfeld's Buchhandlung |location= Dresden, Germany |url=http://digital.slub-dresden.de/fileadmin/data/321768043/321768043_tif/jpegs/321768043.pdf |access-date=15 December 2018 |archive-url=https://web.archive.org/web/20181215223343/http://digital.slub-dresden.de/fileadmin/data/321768043/321768043_tif/jpegs/321768043.pdf |archive-date=15 December 2018 |url-status=dead}} Contemporary with Fallou's work, and driven by the same need to accurately assess land for equitable taxation, Vasily Dokuchaev led a team of soil scientists in Russia who conducted an extensive survey of soils, observing that similar basic rocks, climate and vegetation types lead to similar soil layering and types, and established the concepts for soil classifications. Due to language barriers, the work of this team was not communicated to Western Europe until 1914 through a publication in German by Konstantin Glinka, a member of the Russian team.{{cite book |language=de |last=Glinka |first=Konstantin Dmitrievich |title=Die Typen der Bodenbildung: ihre Klassifikation und geographische Verbreitung |year=1914 |publisher=Borntraeger |location=Berlin, Germany}}

Curtis F. Marbut, influenced by the work of the Russian team, translated Glinka's publication into English,{{cite book |last=Glinka |first=Konstantin Dmitrievich |title=The great soil groups of the world and their development |url=http://reader.library.cornell.edu/docviewer/digital?id=chla3055800#mode/1up |year=1927 |publisher=Edwards Brothers |location=Ann Arbor, Michigan |access-date=19 September 2021}} and, as he was placed in charge of the U.S. National Cooperative Soil Survey, applied it to a national soil classification system.

{{portal|Environment|Geology}}

{{div col|content=

• Acid sulfate soil
• Agricultural science
• Agrophysics
• Crust
• Factors affecting permeability of soils
• Geomorphology
• Index of soil-related articles
• Lunar soil and martian soil
• Mycorrhizal fungi and soil carbon storage
• Red soil
• Shrink–swell capacity
• Soil biodiversity
• Soil liquefaction
• Soil moisture velocity equation
• Soil zoology
• Tillage erosion
• World Soil Museum

}}

{{reflist}}

{{Free-content attribution

| title = Drowning in Plastics – Marine Litter and Plastic Waste Vital Graphics

| publisher = United Nations Environment Programme

| documentURL = https://www.unep.org/resources/report/drowning-plastics-marine-litter-and-plastic-waste-vital-graphics

| license statement URL = https://commons.wikimedia.org/wiki/File:United_Nations_Environment_Programme_Drowning_in_Plastics_%E2%80%93_Marine_Litter_and_Plastic_Waste_Vital_Graphics.pdf

| license = Cc BY-SA 3.0 IGO

}}

{{refbegin}}

• {{cite book |last1=Donahue |first1=Roy Luther |title=Soils: An Introduction to Soils and Plant Growth |last2=Miller |first2=Raymond W. |last3=Shickluna |first3=John C. |year=1977 |publisher=Prentice-Hall |isbn=978-0-13-821918-5 |url=https://archive.org/details/soilsintroductio00dona}}
• {{cite web |title=Arizona Master Gardener |url=http://ag.arizona.edu/pubs/garden/mg/soils/soils.html|publisher=Cooperative Extension, College of Agriculture, University of Arizona |access-date=27 May 2013}}
• {{cite book |editor-last=Stefferud |editor-first=Alfred |title=Soil: The Yearbook of Agriculture 1957 |year=1957 |publisher=United States Department of Agriculture |url=https://archive.org/stream/yoa1957#page/n18/mode/1up |oclc=704186906}}
• {{harvc |name-list-style=harv |last=Kellogg |first=Charles E. |chapter=We Seek; We Learn |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n17/mode/1up}}
• {{harvc |name-list-style=harv |last=Simonson |first=Roy W. |chapter=What Soils Are |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n34/mode/1up}}
• {{harvc |name-list-style=harv |last=Russell |first=M.B. |chapter=Physical Properties |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n49/mode/1up}}
• {{harvc |name-list-style=harv |last=Dean |first=L.A. |chapter=Plant Nutrition and Soil Fertility |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n100/mode/1up}}
• {{harvc |name-list-style=harv |last=Russel |first=Darrell A. |chapter=Boron and Soil Fertility |in=Stefferud |year=1957 |url=//archive.org/stream/yoa1957#page/n145/mode/1up |oclc=704186906}}

{{refend}}

{{refbegin|colwidth=33em}}

• [http://www.soil-net.com/ Soil-Net.com] {{Webarchive|url=https://web.archive.org/web/20080710061716/http://www.soil-net.com/ |date=10 July 2008 }} A free schools-age educational site teaching about soil and its importance.
• Adams, J.A. 1986. Dirt. College Station, Texas: Texas A&M University Press {{ISBN|0-89096-301-0}}
• Certini, G., Scalenghe, R. 2006. Soils: Basic concepts and future challenges. Cambridge Univ Press, Cambridge.
• Montgomery, David R., Dirt: The Erosion of Civilizations (U of California Press, 2007), {{ISBN|978-0-520-25806-8}}
• Faulkner, Edward H. Plowman's Folly (New York, Grosset & Dunlap, 1943). {{ISBN|0-933280-51-3}}
• [https://web.archive.org/web/20080705133103/http://www.landis.org.uk/soilscapes LandIS Free Soilscapes Viewer] Free interactive viewer for the Soils of England and Wales
• Jenny, Hans. 1941. [https://web.archive.org/web/20130225050838/http://soilandhealth.org/01aglibrary/010159.Jenny.pdf Factors of Soil Formation: A System of Quantitative Pedology]
• Logan, W.B. Dirt: The ecstatic skin of the earth (1995). {{ISBN|1-57322-004-3}}
• Mann, Charles C. September 2008. " Our good earth" National Geographic Magazine

• {{cite web|url=http://www.mvm.usace.army.mil/Readiness/97flood/flood.htm |title=97 Flood |publisher=USGS |access-date=8 July 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080624040143/http://www.mvm.usace.army.mil/Readiness/97flood/flood.htm |archive-date=24 June 2008}} Photographs of sand boils.
• Soil Survey Division Staff. 1999. Soil survey manual. Soil Conservation Service. U.S. Department of Agriculture Handbook 18.
• Soil Survey Staff. 1975. Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys. USDA-SCS Agric. Handb. 436. United States Government Printing Office, Washington, DC.
• [https://web.archive.org/web/20060828063956/http://forages.oregonstate.edu/is/ssis/main.cfm?PageID=3 Soils (Matching suitable forage species to soil type)], Oregon State University
• {{cite web|url= http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec1/Lec1.html|title= Lecture 1 Chapter 1 Why Study Soils?|access-date= 7 January 2019|last= Gardiner|first= Duane T|website= ENV320: Soil Science Lecture Notes|publisher= Texas A&M University-Kingsville|archive-url= https://web.archive.org/web/20180209052922/http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec1/Lec1.html|archive-date= 9 February 2018|url-status=dead|df= dmy-all}}
• Janick, Jules. 2002. [https://web.archive.org/web/20050317030248/http://www.hort.purdue.edu/newcrop/tropical/lecture_06/chapter_12l_R.html Soil notes], Purdue University
• [http://www.landis.org.uk/ LandIS Soils Data for England and Wales] {{Webarchive|url=https://web.archive.org/web/20070716033248/http://www.landis.org.uk/ |date=16 July 2007 }} a pay source for GIS data on the soils of England and Wales and soils data source; they charge a handling fee to researchers.

{{refend}}

{{wiktionary|soil}}

{{wikiversity|Soil Formation}}

{{Wikibooks |Historical Geology|Soils and paleosols}}

{{Commons category|Soils}}

{{Wikiquote}}

{{div col|content=

• [https://www.theguardian.com/environment/video/2019/jul/11/its-time-we-stopped-treating-soil-like-dirt-video Short video explaining soil basics]
• [http://www.edaphic.com.au/soil-water-compendium/ The Soil Water Compendium (soil water content sensors explained)]
• [http://www.fao.org/globalsoilpartnership/en/ Global Soil Partnership]
• [http://www.fao.org/soils-portal/en/ FAO Soils Portal]
• [https://wrb.isric.org/ World Reference Base for Soil Resources]
• [https://www.isric.org/ ISRIC – World Soil Information (ISC World Data Centre for Soils)]
• [https://www.isric.org/explore/library ISRIC -World Soil Library and Maps]
• [https://wsm.isric.org/ ISRIC - World Soil Museum (WSM virtual)]
• [https://data.isric.org/ ISRIC - Soil data hub]
• [http://www.wossac.com/ Wossac the world soil survey archive and catalogue]
• [http://csss.ca/ Canadian Society of Soil Science]
• [https://www.soils.org/ Soil Science Society of America]
• [http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm USDA-NRCS Web Soil Survey]
• [http://eusoils.jrc.ec.europa.eu/ European Soil Portal] (wiki)
• [http://www.cranfield.ac.uk/sas/nsri National Soil Resources Institute UK]
• [http://passel.unl.edu/ Plant and Soil Sciences eLibrary]
• [https://archive.org/details/yoa1957 Copies of the reference 'Soil: The Yearbook of Agriculture 1957' in multiple formats]

}}

{{Use dmy dates|date=June 2019}}

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