Editing Soil (section)

===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}}