Editing Soil (section)

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

The chemistry of a soil determines its ability to supply available [[Plant nutrition|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 [[Sorption|absorb]] [[pollutants]] and to filter water. It is the [[surface chemistry]] of mineral and organic [[colloids]] that determines soil's chemical properties.<ref>{{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 }}</ref> A colloid is a small, insoluble particle ranging in size from 1 [[nanometer]] to 1 [[micrometre|micrometer]], thus small enough to remain suspended by [[Brownian motion]] in a fluid medium without settling.<ref>{{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 }}</ref> Most soils contain organic colloidal particles called [[humus]] as well as the inorganic colloidal particles of [[clay]]s. The very high [[specific surface area]] of colloids and their net [[electrical charge]]s 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&nbsp;grams of soil (or centimoles of positive charge per kilogram of soil; [[Cation-exchange capacity|cmol<sub>c</sub>/kg]]). Similarly, positively charged sites on colloids can attract and release [[anions]] in the soil, giving the soil anion exchange capacity.

===Cation and anion exchange===
{{Further|Cation-exchange capacity}}
The cation exchange, that takes place between colloids and soil water, [[Buffer solution|buffers]] (moderates) soil pH, alters soil structure, and purifies [[Percolation|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}}

# Isomorphous substitution occurs in clay during its formation, when lower-valence cations substitute for higher-valence cations in the crystal structure.<ref name="PMID10097044">{{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 }}</ref> 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.
# Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete.<ref>{{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 }}</ref>
# [[Hydroxyl]]s 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).<ref>{{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 }}</ref>
# Hydrogens of humus hydroxyl groups may also be ionised into solution, leaving, similarly to clay, an oxygen with a negative charge.<ref>{{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 }}</ref>

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.<ref>{{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 }}</ref><ref>{{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 }}</ref>

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:

Al<sup>3+</sup> replaces H<sup>+</sup> replaces Ca<sup>2+</sup> replaces Mg<sup>2+</sup> replaces K<sup>+</sup> same as {{chem|NH|4|+}} replaces Na<sup>+</sup><ref>{{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 }}</ref>

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 [[Fertilizer|fertilisers]] ([[potash]], [[Lime (material)|lime]]).<ref>{{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 }}</ref>

As the soil solution becomes more acidic (low [[pH]], meaning an abundance of H<sup>+</sup>), 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 [[Ionization|ionisation]] of [[hydroxy group]]s on the surface of soil colloids creates what is described as pH-dependent [[Surface charge|surface charges]].<ref>{{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 }}</ref> Unlike permanent charges developed by [[Isomorphous replacement|isomorphous substitution]], pH-dependent charges are variable and increase with increasing pH.<ref name="CEC">{{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 }}</ref> Freed cations can be made available to plants but are also prone to be leached from the soil, possibly making the soil less fertile.<ref>{{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 }}</ref> Plants are able to excrete H<sup>+</sup> into the soil through the synthesis of [[organic acid]]s 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.<ref>{{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 }}</ref>

====Cation exchange capacity (CEC)====
[[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.<ref>{{cite journal |last=Brown |first=John C. |year=1978 |title=Mechanism of iron uptake by plants |journal=[[Plant, Cell & Environment|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 }}</ref> CEC is the amount of exchangeable hydrogen cations (H<sup>+</sup>) that will combine with 100&nbsp;grams dry weight of soil and whose measure is one [[milliequivalent]] per 100&nbsp;grams of soil (1&nbsp;meq/100&nbsp;g). Hydrogen ions have a single charge and one-thousandth of a gram of hydrogen ions per 100&nbsp;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 (chemistry)|valence]] of two, converts to {{nowrap|(40 ÷ 2) × 1 milliequivalent}} = 20 milliequivalents of hydrogen ion per 100&nbsp;grams of dry soil or 20&nbsp;meq/100&nbsp;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 rainforest]]s), due to leaching and decomposition, respectively, explains the apparent sterility of tropical soils.<ref>{{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 (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 }}</ref> Live plant roots also have some CEC, linked to their [[specific surface area]].<ref>{{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 }}</ref>

{| 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&nbsp;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 (AEC)====
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.<ref name="Hinsinger 2001 173–195">{{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 }}</ref> Those colloids which have low [[Cation-exchange capacity|CEC]] tend to have some AEC. [[Amorphous solid|Amorphous]] and [[sesquioxide]] clays have the highest AEC,<ref>{{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 }}</ref> followed by the iron oxides.<ref>{{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 (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 }}</ref> 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.<ref>{{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 }}</ref> Phosphates tend to be held at anion exchange sites.<ref>{{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 }}</ref>

Iron and aluminum hydroxide clays are able to exchange their hydroxide anions (OH<sup>−</sup>) for other anions.<ref name="Hinsinger 2001 173–195"/> 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<sup>−</sup>

The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100&nbsp;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).<ref>{{cite journal |last1=Lawrinenko |first1=Mike |last2=Laird |first2=David A. |year=2015 |title=Anion exchange capacity of biochar |journal=[[Green Chemistry (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 }}</ref>

===Reactivity (pH)===
{{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|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.<ref>{{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 }}</ref>

At 25&nbsp;°C an aqueous solution that has a pH of 3.5 has 10<sup>−3.5</sup> [[mole (unit)|moles]] H<sub>3</sub>O<sup>+</sup> (hydronium ions) per litre of solution (and also 10<sup>−10.5</sup> moles per litre OH<sup>−</sup>). A pH of 7, defined as neutral, has 10<sup>−7</sup> moles of hydronium ions per litre of solution and also 10<sup>−7</sup> moles of OH<sup>−</sup> per litre; since the two concentrations are equal, they are said to neutralise each other. A pH of 9.5 has 10<sup>−9.5</sup> moles hydronium ions per litre of solution (and also 10<sup>−2.5</sup> moles per litre OH<sup>−</sup>). 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 10<sup>6</sup>) and is more acidic.<ref>{{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 }}</ref>

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]].<ref>{{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 }}</ref> As a result of a trade-off between toxicity and requirement most nutrients are better available to plants at moderate pH,<ref>{{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 }}</ref> 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&nbsp;6.5 and organic soils of pH&nbsp;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,<ref>{{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 }}</ref><ref>{{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 }}</ref> 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.<ref>{{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 }}</ref>

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 [[Acid rain|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]].<ref>{{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 }}</ref> Once the colloids are saturated with H<sub>3</sub>O<sup>+</sup>, 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]].<ref>{{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 }}</ref> In areas of extreme rainfall and high temperatures, the clay and humus may be washed out, further reducing the buffering capacity of the soil.<ref>{{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 }}</ref> In low rainfall areas, unleached calcium pushes pH to 8.5 and with the addition of exchangeable sodium, soils may reach pH&nbsp;10.<ref>{{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 }}</ref> Beyond a pH of 9, plant growth is reduced.<ref>{{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 (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 }}</ref> High pH results in low [[micro-nutrient]] mobility, but water-soluble [[chelates]] of those nutrients can correct the deficit.<ref>{{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 (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 }}</ref> 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}}<ref>{{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 }}</ref>

==== Base saturation percentage ====
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&nbsp;meq and 5&nbsp;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).<ref>{{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, & Soil Pollution|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 }}</ref> 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).<ref>{{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 }}</ref> 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}}

====Buffering====
{{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 [[Cation-exchange capacity|CEC]] increases. Hence, pure sand has almost no buffering ability, though soils high in [[Colloid|colloids]] (whether mineral or organic) have high buffering capacity.<ref>{{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 }}</ref> Buffering occurs by cation exchange and [[Neutralization (chemistry)|neutralisation]]. However, colloids are not the only regulators of soil pH. The role of [[carbonates]] should be underlined, too.<ref>{{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 }}</ref> More generally, according to pH levels, several buffer systems take precedence over each other, from [[calcium carbonate]] [[buffer range]] to iron buffer range.<ref>{{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 }}</ref>

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 [[liming (soil)|lime]], Ca(OH)<sub>2</sub>, will displace hydronium ions from the soil colloids, causing the fixation of calcium to colloids and the evolution of CO<sub>2</sub> 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 [[Cation-exchange capacity|CEC]] of the soil; the greater the CEC, the greater the buffering capacity of the soil.{{sfn|Donahue|Miller|Shickluna|1977|pp=120–121}}

=== Redox ===
{{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]]. [[Redox|Oxidation]] occurs if there is a loss of electrons in the transfer process while [[Redox|reduction]] occurs if there is a gain of electrons. [[Reduction potential]] is measured in volts or millivolts. Soil microbial communities develop along [[electron transport chain]]s, forming electrically conductive [[Geobacter#Biofilm conductivity|biofilms]], and developing networks of [[bacterial nanowires]].

Redox factors act on soil development, with [[redoximorphic features|redoximorphic color features]] providing critical information for soil interpretation. Understanding the [[Redox gradient#Terrestrial Environments|redox gradient]] is important to managing [[carbon sequestration]], [[bioremediation]], [[Pedosphere#Redox conditions in wetland soils|wetland delineation]], and [[soil-based microbial fuel cell]]s.