View source for Soil pH

{{Short description|Measure of how acidic or alkaline the soil is}}
{{Redirect-distinguish|Mediacid|Medicaid}}
{{Use British English|date=February 2021}}

[[File:World Soil pH.svg|thumb|343px|right|Global variation in soil pH. '''<span style="color:red;">Red</span>''' = acidic soil. '''<span style="color:yellow;">Yellow</span>''' = neutral soil. '''<span style="color:blue;">Blue</span>''' = alkaline soil. '''Black''' = no data.]]
'''Soil pH''' is a measure of the [[acidity]] or [[basicity]] (alkalinity) of a [[soil]]. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics.<ref>{{cite book |doi=10.2136/sssabookser5.3.c16 |chapter=Soil pH and soil acidity |title=Methods of soil analysis |series=SSSA Book Series |issn=2163-2804 |year=1996 |last=Thomas |first=Grant W. |pages=475–90 |isbn=978-0-89118-866-7 |s2cid=93493509 |chapter-url=https://fr.1lib.sk/book/95586147/8544e6 |access-date=14 July 2025 |editor-last1=Sparks |editor-first1=Donald L. |editor-last2=Page |editor-first2=A. L. |editor-last3=Helmke |editor-first3=Philip August |editor-last4=Loeppert |editor-first4=Richard H. |editor-last5=Soltanpour |editor-first5=Parviz N. |editor-last6=Tabatabai |editor-first6=M. Ali |editor-last7=Johnston |editor-first7=Cliff T. |editor-last8=Sumner |editor-first8=Malcolm E. |publisher=[[Soil Science Society of America]] |location=Madison, Wisconsin }}</ref> [[pH]] is defined as the negative [[logarithm]] (base&nbsp;10) of the [[activity (chemistry)|activity]] of [[hydronium]] ions ({{chem|H|+}} or, more precisely, {{chem|H|3|O|+|aq}}) in a [[aqueous solution|solution]]. In soils, it is measured in a slurry of soil mixed with water (or a salt solution, such as {{val|0.01|ul=M}}&nbsp;{{chem|Ca|Cl|2}}), and normally falls between 3 and 10, with 7 being neutral. Acid soils have a pH below 7 and [[Alkali soil|alkaline soils]] have a pH above 7. Ultra-acidic soils (pH < 3.5) and very strongly alkaline soils (pH > 9) are rare.<ref name="Slessarev2017">{{cite journal|last1=Slessarev |first1=Eric W. |last2=Lin |first2=Yuan |last3=Bingham |first3=Nina L. |last4=Johnson |first4=Jennifer E. |last5=Dai |first5=Yongjiu
 |last6=Schimel |first6=Joshua P. |last7=Chadwick |first7=Oliver A. |title=Water balance creates a threshold in soil pH at the global scale |journal=[[Nature (journal)|Nature]] |date=21 November 2016 |volume=540 |issue=7634 |pages=567–69 |doi=10.1038/nature20139 |pmid=27871089 |bibcode=2016Natur.540..567S |s2cid=4466063 |url=https://escholarship.org/content/qt30f631wk/qt30f631wk.pdf |access-date=5 February 2023 }}</ref><ref name="QueenslandGovt2017">{{cite web |last=Queensland Government |title=Soil pH |url=https://www.qld.gov.au/environment/land/soil/soil-properties/ph-levels/ |publisher=[[Queensland Government]] |access-date=5 February 2023 |language=en-AU}}</ref>

Soil pH is considered a master variable in soils as it affects many chemical processes. It specifically affects [[Plant nutrition|plant nutrient]] availability by controlling the chemical forms of the different nutrients and influencing the chemical reactions they undergo. The optimum pH range for most plants is between 5.5 and 7.5;<ref name="QueenslandGovt2017"/> however, many plants have adapted to thrive at pH&nbsp;values outside this range.

==Classification of soil pH ranges==

The [[United States Department of Agriculture]] [[Natural Resources Conservation Service]] classifies soil pH ranges as follows:<ref>{{cite web |author=Soil Science Division Staff |url=https://www.nrcs.usda.gov/sites/default/files/2022-09/SSM-ch3.pdf |title=Soil Survey Manual 2017, Chapter 3, Examination and description of soil profiles |publisher=[[Natural Resources Conservation Service]], United States Department of Agriculture, Handbook 18 |access-date=12 February 2023}}</ref>

{|class="wikitable" style="align: center;"
!Semantic description
!pH range
|-
|Ultra acidic|| < 3.5
|-
|Extremely acidic|| 3.5–4.4
|-
|Very strongly acidic|| 4.5–5.0 
|-
|Strongly acidic|| 5.1–5.5
|-
|Moderately acidic|| 5.6–6.0
|-
|Slightly acidic|| 6.1–6.5
|-
|Neutral|| 6.6–7.3
|-
|Slightly alkaline|| 7.4–7.8
|-
|Moderately alkaline|| 7.9–8.4
|-
|Strongly alkaline|| 8.5–9.0
|-
|Very strongly alkaline|| > 9.0
|-
|}
0 to 6=acidic<br />7=neutral<br />8 and above=alkaline

==Determining pH==
Methods of determining pH include:
*Observation of soil profile: certain profile characteristics can be indicators of either acid, saline, or sodic conditions.  Examples are:<ref>{{cite book |isbn=978-0-8138-2873-2 |title=Soil genesis and classification |edition=Fifth |year=2003 |access-date=12 February 2023 |url=https://archive.org/details/soilgenesisclass0000unse_h4i0 |editor-last1=Buol |editor-first1=Stanley W. |editor-last2=Southard |editor-first2=Randal J. |editor-last3=Graham |editor-first3=Robert C. |editor-last4=McDaniel |editor-first4=Paul A. |publisher=[[Wiley-Blackwell|Wiley–Blackwell]] |location=Hoboken, New Jersey }}</ref>
**Poor incorporation of the organic surface layer with the underlying mineral layer – this can indicate strongly acidic soils;
**The classic [[podzol]] [[Soil horizon|horizon]] sequence, since podzols are strongly acidic: in these soils, a pale eluvial (E) horizon lies under the organic surface layer and overlies a dark B horizon;
**Presence of a [[caliche]] layer indicates the presence of calcium carbonates, which are present in alkaline conditions;
**Columnar [[Soil structure|structure]] can be an indicator of [[Soil salinity|sodic]] condition.
*Observation of predominant flora. [[Calcifuge]] plants (those that prefer an acidic soil) include ''[[Erica (plant)|Erica]]'', ''[[Rhododendron]]'' and nearly all other [[Ericaceae]] species, many [[birch]] (''Betula''), foxglove (''[[Digitalis]]''), [[gorse]] (''Ulex'' spp.), and [[Scots Pine]] (''Pinus sylvestris''). [[Calcicole]] (lime loving) plants include ash trees (''[[Fraxinus]]'' spp.), [[honeysuckle]] (''Lonicera''), ''[[Buddleja]]'', dogwoods (''[[Cornus (plant)|Cornus]]'' spp.), lilac (''[[Syringa]]'') and ''[[Clematis]]'' species.
*Use of an inexpensive pH testing kit, where in a small sample of soil is mixed with [[pH indicator|indicator]] solution which changes colour according to the acidity.
*Use of [[litmus paper]]. A small sample of soil is mixed with distilled water, into which a strip of litmus paper is inserted. If the soil is acidic the paper turns red, if basic, blue.
*Certain other fruit and vegetable pigments also change color in response to changing pH. [[Blueberry]] juice turns more reddish if acid is added, and becomes indigo if titrated with sufficient base to yield a high pH. Red [[cabbage]] is similarly affected.
*Use of a commercially available electronic [[pH meter]], in which a glass or solid-state [[electrode]] is inserted into moistened soil or a mixture (suspension) of soil and water; the pH is usually read on a digital display screen.<ref>{{Cite web |title=Evolution of the pH Meter |url=https://www.labmanager.com/evolution-of-the-ph-meter-19282 |access-date=2023-09-22 |website=Lab Manager |language=en}}</ref>
* In the 2010s, [[Spectrophotometry|spectrophotometric]] methods were developed to measure soil pH involving addition of an indicator dye to the soil extract.<ref>{{cite journal |last1=Bargrizan |first1=Sima |last2=Smernik |first2=Ronald J. |last3=Mosley |first3=Luke M. |title=Development of a spectrophotometric method for determining pH of soil extracts and comparison with glass electrode measurements |journal=[[Soil Science Society of America Journal]] |date=November 2017 |volume=81 |issue=6 |pages=1350–58 |doi=10.2136/sssaj2017.04.0119 |bibcode=2017SSASJ..81.1350B |url=https://www.researchgate.net/publication/318960029 |access-date=12 February 2023}}</ref> These compare well to [[glass electrode]] measurements but offer substantial advantages such as lack of drift, liquid junction and suspension effects.

Precise, repeatable measures of soil pH are required for scientific research and monitoring. This generally entails laboratory analysis using a standard protocol; an example of such a protocol is that in the [[United States Department of Agriculture|USDA]] Soil Survey Field and Laboratory Methods Manual.<ref name="USDA2014">{{cite book |last1=Soil Survey Staff |editor1-last=Rebecca Burt and Soil Survey Staff |title=Kellogg Soil Survey Laboratory Methods Manual. Soil Survey Investigations Report No. 42, Version 5.0 |date=2014 |publisher=United States Department of Agriculture, Natural Resources Conservation Service |pages=276–279 |url=https://data.neonscience.org/documents/10179/2357445/KelloggSSL_MethodsManual_Report42Version5_2014/da9589dd-3278-402b-a5d4-02dc0c9c762c |access-date=19 February 2023}}</ref> In this document the three-page protocol for soil pH measurement includes the following sections: Application; Summary of Method; Interferences; Safety; Equipment; Reagents; and Procedure.{{Quote frame |quote=
Summary of Method
<poem>
The pH is measured in soil-water (1:1) and soil-salt (1:2 <chem>CaCl2</chem>) solutions. For convenience, the pH is initially measured in water and then measured in <chem>CaCl2</chem>. With the addition of an equal volume of 0.02 M <chem>CaCl2</chem> to the soil suspension that was prepared for the water pH, the final soil-solution ratio is 1:2 0.01 M <chem>CaCl2</chem>.
A 20-g soil sample is mixed with 20 mL of reverse osmosis (RO) water (1:1 w:v) with occasional stirring. The sample is allowed to stand 1 h with occasional stirring. The sample is stirred for 30 s, and the 1:1 water pH is measured. The 0.02 M <chem>CaCl2</chem> (20 mL) is added to soil suspension, the sample is stirred, and the 1:2 0.01 M <chem>CaCl2</chem> pH is measured (4C1a2a2).
</poem>
|align=center
|source=Summary of the USDA NRCS method for soil pH determination<ref name="USDA2014"/>}}

==Factors affecting soil pH==
The pH of a natural soil depends on the mineral composition of the [[parent material]] of the soil, and the [[weathering]] reactions undergone by that parent material. In warm, humid environments, [[soil acidification]] occurs over time as the products of weathering are leached by water moving laterally or downwards through the soil. In dry climates, however, soil weathering and leaching are less intense and soil pH is often neutral or alkaline.<ref name="USDA-NRCS">{{cite web |last1=USDA-NRCS |title=Soil pH |url=https://www.nrcs.usda.gov/sites/default/files/2022-10/Soil%20PH.pdf |website=Soil Health - Guides for Educators |access-date=19 February 2023}}</ref><ref>{{cite journal |last1=Van Breemen |first1=Nico |author-link1=Nico van Breemen |last2=Mulder |first2=Jan |last3=Driscoll |first3=Charles T.|title=Acidification and alkalinization of soils |journal=[[Plant and Soil]] |date=October 1983 |volume=75 |issue=3 |pages=283–308 |doi=10.1007/BF02369968 |bibcode=1983PlSoi..75..283V |s2cid=39568100 |url=https://www.researchgate.net/publication/40163032 |access-date=19 February 2023}}</ref>

=== Sources of acidity ===
Many processes contribute to soil acidification. These include:<ref name="VanBreemen1984">{{cite journal |last1=Van Breemen |first1=Nico |last2=Driscoll |first2=Charles T. |last3=Mulder |first3=Jan |title=Acidic deposition and internal proton sources in acidification of soils and waters |journal=[[Nature (journal)|Nature]] |date=16 February 1984 |volume=307 |issue=5952 |pages=599–604 |doi=10.1038/307599a0 |bibcode=1984Natur.307..599B |s2cid=4342985 |url=https://www.researchgate.net/publication/40164555 |access-date=19 February 2023}}</ref>
* Rainfall:  Average rainfall has a pH of 5.6 and is moderately acidic due to dissolved atmospheric carbon dioxide ({{chem|link=carbon dioxide|C|O|2}}) that combines with water to form [[carbonic acid]] ({{chem|H|2|C|O|3}}).  When this water flows through the soil it results in the leaching of basic cations as [[Bicarbonate|bicarbonates]]; this increases the percentage of {{chem|Al|3+}} and {{chem|H|+}} relative to other cations.<ref>{{cite web |last=US EPA |first=OAR |date=2016-02-09 |title=What is acid rain?|url=https://www.epa.gov/acidrain/what-acid-rain |access-date=26 February 2023 |website=[[United States Environmental Protection Agency|US EPA]] |language=en}}</ref>
* Root [[Respiration (physiology)|respiration]] and decomposition of organic matter by microorganisms release {{chem|C|O|2}} which increases the carbonic acid ({{chem|H|2|C|O|3}}) concentration and subsequent leaching.
* Plant growth: Plants take up nutrients in the form of ions (e.g. {{chem|N|O|3|-}}, {{chem|N|H|4|+}}, {{chem|Ca|2+}}, {{chem|H|2|P|O|4|-}}), and they often take up more [[cation]]s than [[anion]]s. However, plants must maintain a neutral charge in their roots. In order to compensate for the extra positive charge, they will release {{chem|H|+}} ions from the root. Some plants also exude organic acids into the soil to acidify the zone around their roots to help solubilize metal nutrients that are insoluble at neutral pH, such as iron (Fe).
* Fertilizer use: [[Ammonium]] ({{chem|N|H|4|+}}) fertilizers react in the soil by the process of [[nitrification]] to form nitrate ({{chem|N|O|3|-}}), and in the process release {{chem|H|+}} ions.
* [[Acid rain]]: The burning of fossil fuels releases oxides of sulfur and nitrogen into the atmosphere. These react with water in the atmosphere to form [[sulfuric acid|sulfuric]] and [[nitric acid]] in rain.
* [[Weathering#Oxidation|Oxidative weathering]]: Oxidation of some primary minerals, especially [[Sulfide|sulfides]] and those containing {{chem|Fe|2+}}, generate acidity. This process is often accelerated by human activity: 
** [[Acid mine drainage|Mine spoil]]: Severely acidic conditions can form in soils near some mine spoils due to the oxidation of [[pyrite]].
** [[Acid sulfate soil]]s formed naturally in [[Waterlogging (agriculture)|waterlogged]] coastal and estuarine environments can become highly acidic when drained or excavated.

=== Sources of alkalinity ===
Total soil alkalinity increases with:<ref name="Bloom2012">{{cite book |last1=Bloom |first1=Paul R. |last2=Skyllberg |first2=Ulf |editor1-last=Huang |editor1-first=Pan Ming |editor2-last=Li |editor2-first=Yuncong |editor3-last=Sumner |editor3-first=Malcolm E. |title=Handbook of soil sciences: properties and processes |date=2012 |publisher=[[CRC Press]] |location=Boca Raton, Florida |isbn=978-0-429-09598-6 |doi=10.1201/b11267 |pages=19–1 to 19–14 |edition=2nd |chapter=Soil pH and pH buffering |url=https://lib-nhwjx7zkiyjoeipfyn7m37ed.late.re/book/6097110/94f24a |access-date=12 March 2023}}</ref><ref name="Oosterbaan2003">{{cite web |last1=Oosterbaan |first1=Roland J.|title=Soil alkalinity (alkaline-sodic soils) |url=https://www.waterlog.info/pdf/acidalka.pdf |website=www.waterlog.info |access-date=12 March 2023}}</ref>
* Weathering of [[Silicate mineral|silicate]], [[aluminosilicate]] and [[carbonate mineral|carbonate]] minerals containing {{chem|Na|+}}, {{chem|Ca|2+}}, {{chem|Mg|2+}} and {{chem|K|+}};
* Addition of silicate, aluminosilicate and carbonate minerals to soils; this may happen by deposition of material eroded elsewhere by wind or water, or by mixing of the soil with less weathered material (such as the [[Liming (soil)|addition of limestone]] to acid soils);{{cn|date=August 2024}}
* Addition of water containing dissolved [[bicarbonate]]s (as occurs when [[Irrigation|irrigating]] with high-bicarbonate waters).{{cn|date=August 2024}}

The accumulation of alkalinity in a soil (as carbonates and bicarbonates of Na, K, Ca and Mg) occurs when there is insufficient water flowing through the soils to leach soluble salts. This may be due to arid conditions, or poor internal soil [[drainage]]; in these situations most of the water that enters the soil is transpired (taken up by plants) or evaporates, rather than flowing through the soil.<ref name="Bloom2012"/>

The soil pH usually increases when the total [[alkalinity]] increases, but the balance of the added cations also has a marked effect on the soil pH. For example, increasing the amount of sodium in an alkaline soil tends to induce dissolution of [[calcium carbonate]], which increases the pH. [[Calcareous]] soils may vary in pH from 7.0 to 9.5, depending on the degree to which {{chem|Ca|2+}} or {{chem|Na|+}} dominate the soluble cations.<ref name="Bloom2012"/>

== Effect of soil pH on plant growth ==

=== Acid soils ===
High levels of aluminium occur near mining sites; small amounts of aluminium are released to the environment at the coal-fired power plants or [[Incineration|incinerators]].<ref name="ATSDR Public Health">{{cite web |url=https://www.atsdr.cdc.gov/ToxProfiles/tp22-c1-b.pdf |title=Public health statement for aluminum |website=www.atsdr.cdc.gov |language=en |date=September 2008 |access-date=12 March 2023 |archive-date=12 December 2016 |archive-url=https://web.archive.org/web/20161212212014/https://www.atsdr.cdc.gov/phs/phs.asp?id=1076&tid=34 |url-status=live}}</ref> Aluminium in the air is washed out by the rain or normally settles down but small particles of aluminium remain in the air for a long time.<ref name="ATSDR Public Health" />

Acidic [[precipitation]] is the main natural factor to mobilize aluminium from natural sources<ref name="Piero3">{{cite journal |last=Dolara |first=Piero |date=21 July 2014 |title=Occurrence, exposure, effects, recommended intake and possible dietary use of selected trace compounds (aluminium, bismuth, cobalt, gold, lithium, nickel, silver) |journal=[[International Journal of Food Sciences and Nutrition]] |volume=65 |issue=8 |pages=911–24 |doi=10.3109/09637486.2014.937801 |issn=1465-3478 |pmid=25045935 |s2cid=43779869 |url=https://lib-nhwjx7zkiyjoeipfyn7m37ed.late.re/book/35772390/5dc19c |access-date=12 March 2023|url-access=subscription }}</ref> and the main reason for the environmental effects of aluminium;<ref name="RosselandEldhuset1990">{{cite journal |last1=Rosseland |first1=Bjorn Olav |last2=Eldhuset |first2=Toril Drabløs |last3=Staurnes |first3=Magne |year=1990 |title=Environmental effects of aluminium |journal=Environmental Geochemistry and Health |volume=12 |issue=1–2 |pages=17–27 |doi=10.1007/BF01734045 |pmid=24202562 |bibcode=1990EnvGH..12...17R |s2cid=23714684 |issn=0269-4042 |url=https://www.researchgate.net/publication/258348898 |access-date=12 March 2023}}</ref> however, the main factor of presence of aluminium in salt and freshwater are the industrial processes that also release aluminium into air.<ref name="Piero3" /> Plants grown in acid soils can experience a variety of stresses including [[aluminium]]&nbsp;(Al), [[hydrogen]]&nbsp;(H), and/or [[manganese]]&nbsp;(Mn) toxicity, as well as nutrient deficiencies of [[calcium]]&nbsp;(Ca) and [[magnesium]]&nbsp;(Mg).<ref>{{cite book |title=The nature and properties of soils, global edition |edition=15th |year=2016 |last1=Weil |first1=Raymond R. |last2=Brady |first2=Nyle C. |isbn=978-1-292-16223-2 |url=https://lib-nhwjx7zkiyjoeipfyn7m37ed.late.re/book/3515307/f40531 |access-date=19 March 2023 |publisher=[[Pearson Education]] |location=London, United Kingdom}}</ref>

[[Aluminium#Environmental effects|Aluminium toxicity]] is the most widespread problem in acid soils.  Aluminium is present in all soils to varying degrees, but dissolved Al<sup>3+</sup> is toxic to plants; Al<sup>3+</sup> is most soluble at low pH; above pH&nbsp;5.0, there is little Al in soluble form in most soils.<ref name="Kopittke2016">{{cite journal |last1=Kopittke |first1=Peter M. |last2=Menzies |first2=Neal W. |last3=Wang |first3=Peng |last4=Blamey |first4=F. Pax C. |title=Kinetics and nature of aluminium rhizotoxic effects: a review |journal=[[Journal of Experimental Botany]] |date=August 2016 |volume=67 |issue=15 |pages=4451–67 |doi=10.1093/jxb/erw233 |pmid=27302129 |url=https://articles-pdjrj4ripo5k3r4j2r4mhmo5.late.re/book/55935402/d61ab1 |access-date=19 March 2023|url-access=subscription }}</ref><ref>{{cite journal |last1=Hansson |first1=Karna |last2=Olsson |first2=Bengt A. |last3=Olsson |first3=Mats |last4=Johansson |first4=Ulf |last5=Kleja |first5=Dan Berggren |title=Differences in soil properties in adjacent stands of Scots pine, Norway spruce and silver birch in SW Sweden |journal=[[Forest Ecology and Management]] |date=August 2011 |volume=262 |issue=3 |pages=522–30 |doi=10.1016/j.foreco.2011.04.021 |bibcode=2011ForEM.262..522H |url=https://www.academia.edu/16589041 |access-date=19 March 2023}}</ref> Aluminium is not a plant nutrient, and as such, is not actively taken up by the plants, but enters plant roots passively through [[osmosis]]. Aluminium can exist in many different forms and is a responsible agent for limiting growth in various parts of the world. Aluminium tolerance studies have been conducted in different plant species to see viable thresholds and concentrations exposed along with function upon exposure.<ref>{{cite journal |last1=Wright |first1=Robert J. |title=Soil aluminum toxicity and plant growth |journal=Communications in Soil Science and Plant Analysis |date=September 1989 |volume=20 |issue=15–16 |pages=1479–97 |doi=10.1080/00103628909368163 |bibcode=1989CSSPA..20.1479W |url=https://articles-pdjrj4ripo5k3r4j2r4mhmo5.late.re/book/21233659/9e6a73 |access-date=19 March 2023|url-access=subscription }}</ref> Aluminium inhibits root growth; lateral roots and root tips become thickened and roots lack fine branching; root tips may turn brown. In the root, the initial effect of Al<sup>3+</sup> is the inhibition of the expansion of the cells of the [[rhizodermis]], leading to their rupture; thereafter it is known to interfere with many physiological processes including the uptake and transport of calcium and other essential nutrients, cell division, cell wall formation, and enzyme activity.<ref name="Kopittke2016"/><ref>{{cite journal |last1=Rout |first1=Gyana Ranjan |last2=Samantaray |first2=Sanghamitra |last3=Das |first3=Premananda |title=Aluminium toxicity in plants: a review |journal=Agronomie |date=January 2001 |volume=21 |issue=1 |pages=3–21 |doi=10.1051/agro:2001105 |bibcode=2001AgSD...21....3R |url=https://hal.science/hal-00886101/document |access-date=19 March 2023}}</ref>

Proton (H<sup>+</sup> ion) stress can also limit plant growth. The [[proton pump]], H<sup>+</sup>-ATPase, of the [[plasmalemma]] of root cells works to maintain the near-neutral pH of their [[cytoplasm]]. A high proton activity (pH within the range 3.0–4.0 for most plant species) in the external growth medium overcomes the capacity of the cell to maintain the cytoplasmic pH and growth shuts down.<ref name="Shavrukov2016">{{cite journal |last1=Shavrukov |first1=Yuri |last2=Hirai |first2=Yoshihiko |title=Good and bad protons: genetic aspects of acidity stress responses in plants |journal=[[Journal of Experimental Botany]] |date=January 2016 |volume=67 |issue=1 |pages=15–30 |doi=10.1093/jxb/erv437 |pmid=26417020 |doi-access=free}}</ref>

In soils with a high content of [[manganese]]-containing minerals, Mn toxicity can become a problem at pH 5.6 and lower.  Manganese, like aluminium, becomes increasingly soluble as pH drops, and Mn toxicity symptoms can be seen at pH levels below 5.6.  Manganese is an essential plant nutrient, so plants transport Mn into leaves.  Classic symptoms of Mn toxicity are crinkling or cupping of leaves.<ref>{{cite journal |last=Ramakrishnan |first=Palayanoor Sivaswamy |title=Nutritional requirements of the edaphic ecotypes in ''Melilotus alba'' Medic. II. Aluminium and manganese |journal=[[New Phytologist]] |date=April 1968 |volume=67 |issue=2 |pages=301–08 |doi=10.1111/j.1469-8137.1968.tb06385.x |doi-access=free}}</ref>

===Nutrient availability in relation to soil pH===
[[File:Soil-pH.svg|thumb|right|Nutrient availability in relation to soil pH<ref>{{cite book |first=Arnold |last=Finck |location=Kiel, Germany |year=1976 |title=Pflanzenernährung in Stichworten |publisher=Hirt |isbn=978-3-554-80197-2 |page=80}}</ref>]]

Soil pH affects the availability of some [[Plant nutrition|plant nutrients]]:

As discussed above, aluminium toxicity has direct effects on plant growth; however, by limiting root growth, it also reduces the availability of plant nutrients. Because roots are damaged, nutrient uptake is reduced, and deficiencies of the [[macronutrients]] (nitrogen, phosphorus, potassium, calcium and magnesium) are frequently encountered in very strongly acidic to ultra-acidic soils (pH<5.0).<ref name="Sumner2002">{{cite journal |last1=Sumner |first1=Malcolm E. |last2=Yamada |first2=Tsuioshi |title=Farming with acidity |journal=Communications in Soil Science and Plant Analysis |date=November 2002 |volume=33 |issue=15–18 |pages=2467–96 |doi=10.1081/CSS-120014461 |bibcode=2002CSSPA..33.2467S |s2cid=93165895}}</ref> When aluminum levels increase in the soil, it decreases the pH levels. This does not allow for trees to take up water, meaning they cannot photosynthesize, leading them to die. The trees can also develop yellowish colour on their leaves and veins.<ref>{{cite journal |last1=Cape |first1=J. N. |title=Direct damage to vegetation caused by acid rain and polluted cloud: definition of critical levels for forest trees |journal=[[Environmental Pollution (journal)|Environmental Pollution]] |date=1 January 1993 |volume=82 |issue=2 |pages=167–180 |doi=10.1016/0269-7491(93)90114-4 |pmid=15091786 |url=https://articles-pdjrj4ripo5k3r4j2r4mhmo5.late.re/book/19978680/d768f2 |access-date=2 April 2023|url-access=subscription }}</ref>

[[Plant nutrition#Molybdenum|Molybdenum]] availability is increased at higher pH; this is because the molybdate ion is more strongly sorbed by clay particles at lower pH.<ref name="Huang2011">{{cite book |last1=Bolan |first1=Nanthi |last2=Brennan |first2=Ross |last3=Budianta |first3=Dedik |last4=Camberato |first4=James J. |last5=Naidu |first5=Ravi |last6=Pan |first6=William L. |last7=Sharpley |first7=Andrew |last8=Sparks |first8=Donald L. |last9=Sumner |first9=Malcolm E. |editor1-last=Huang |editor1-first=Pan Ming |editor2-last=Li |editor2-first=Yuncong |editor3-last=Sumner |editor3-first=Malcolm E. |title=Handbook of soil sciences: resource management and environmental impacts |date=2012 |publisher=[[CRC Press]] |location=Boca Raton, Florida |isbn=978-1-4398-0308-0 |pages=11–1 to 11–80 |edition=2nd |chapter=Bioavailability of N, P, K, Ca, Mg, S, Si, and micronutrients |url=https://lib-nhwjx7zkiyjoeipfyn7m37ed.late.re/book/2214335/a4c9a4 |access-date=2 April 2023}}</ref>

[[Plant nutrition#Zinc|Zinc]], [[Plant nutrition#Iron|iron]], [[Plant nutrition#Copper|copper]] and [[Plant nutrition#Manganese|manganese]] show decreased availability at higher pH (increased [[sorption]] at higher pH).<ref name="Huang2011"/>

The effect of pH on [[Plant nutrition#Phosphorus|phosphorus]] availability varies considerably, depending on soil conditions and the crop in question. The prevailing view in the 1940s and 1950s was that P availability was maximized near neutrality (soil pH 6.5–7.5), and decreased at higher and lower pH.<ref name="Truog1946">{{cite book |last1=Truog |first1=Emil |title=Science in farming, USDA Yearbook, 1941–1947 |date=1946 |pages=566–76 |chapter=The liming of soils |chapter-url=https://naldc.nal.usda.gov/download/IND43893966/PDF |access-date=9 April 2023}}</ref><ref name="Sumner1986">{{cite book |last1=Sumner |first1=Malcolm E. |last2=Farina |first2=Mart P.W. |editor1-last=Stewart |editor1-first=Bob A. |title=Advances in soil science |date=1986 |publisher=[[Springer Publishing|Springer]] |location=New York, New York |isbn=978-1-4613-8660-5 |pages=201–36 |chapter=Phosphorus interactions with other nutrients and lime in field cropping systems |doi=10.1007/978-1-4613-8660-5_5 |chapter-url=https://www.researchgate.net/publication/286120267 |access-date=9 April 2023}}</ref> Interactions of phosphorus with pH in the moderately to slightly acidic range (pH 5.5–6.5) are, however, far more complex than is suggested by this view. Laboratory tests, glasshouse trials and field trials have indicated that increases in pH within this range may increase, decrease, or have no effect on P availability to plants.<ref name="Sumner1986"/><ref name="Haynes1982">{{cite journal |last1=Haynes |first1=R. J. |title=Effects of liming on phosphate availability in acid soils: a critical review |journal=[[Plant and Soil]] |date=October 1982 |volume=68 |issue=3 |pages=289–308 |doi=10.1007/BF02197935 |bibcode=1982PlSoi..68..289H |s2cid=22695096 |url=https://articles-pdjrj4ripo5k3r4j2r4mhmo5.late.re/book/6871474/3b010d |access-date=9 April 2023|url-access=subscription }}</ref>

== Water availability in relation to soil pH ==
{{Further|Water content|Water potential|Alkali soil}}
Strongly alkaline soils are [[Sodic soil|sodic]] and [[Dispersion (geology)|dispersive]], with slow [[Infiltration (hydrology)|infiltration]], low [[hydraulic conductivity]] and poor [[available water capacity]].<ref name="Ellis Foth 2017">{{cite book |last1=Ellis |first1=Boyd |last2=Foth |first2=Henry |title=Soil fertility |edition=2nd |pages=73–74 |date=1997 | url=https://lib-nhwjx7zkiyjoeipfyn7m37ed.late.re/book/3580248/0f9606 |access-date=10 April 2023 |isbn=978-1-56670-243-0 |publisher=[[CRC Press]] |location=Boca Raton, Florida }}</ref> Plant growth is severely restricted because [[aeration]] is poor when the soil is wet; while in dry conditions, plant-available water is rapidly depleted and the soils become hard and cloddy (high soil strength).<ref>{{cite web |title=Sodic soils and plant growth |url=https://www.fao.org/3/x5871e/x5871e05.htm#4.3%20Sodic%20soils%20and%20plant%20growth |website=fao.org |access-date=10 April 2023 |language=en}}</ref> The higher the pH in the soil, the less water available to be distributed to the plants and organisms that depend on it. With a decreased pH, this does not allow for plants to uptake water like they normally would. This causes them to not be able to photosynthesize.<ref>{{cite journal |doi=10.1155/2019/5794869 |doi-access=free |title=The role of soil pH in plant nutrition and soil remediation |year=2019 |last1=Neina |first1=Dora |journal=[[Applied and Environmental Soil Science]] |volume=2019 |issue=1 |pages=1–9 |bibcode=2019ApESS201994869N }}</ref>

Many strongly acidic soils, on the other hand, have strong aggregation, good [[drainage|internal drainage]], and good water-holding characteristics. However, for many plant species, [[Aluminium#Effect on plants|aluminium toxicity]] severely limits root growth, and [[moisture stress]] can occur even when the soil is relatively moist.<ref name="Kopittke2016"/>

==Plant pH preferences==
In general terms, different plant species are adapted to soils of different pH ranges. For many species, the suitable soil pH range is fairly well known.<ref>{{Cite web |title=Optimal Soil pH Levels for Vegetable Farming |url=https://www.agriculturelandusa.com/2024/10/optimal-soil-ph-levels-for-vegetable-farming.html |access-date=2024-10-21 |website=Agriculture land usa |language=en-US}}</ref> Online databases of plant characteristics, such as ''USDA PLANTS''<ref name="USDA2017">{{cite web |last1=USDA NRCS |title=Plants Database Characteristics Search |url=https://plants.usda.gov/home/characteristicsSearchResults?resultId=6e52d145-7c59-4537-b815-dfd960df195c |website=plants.usda.gov |publisher=[[Natural Resources Conservation Service|USDA NRCS]] |access-date=16 April 2023 |date=2023}}</ref> and ''Plants for a Future''<ref name="PFAF2017">{{cite web |last1=Plants for a future |title=Plant Database Search Page |url=https://pfaf.org/USER/plantsearch.aspx |website=www.pfaf.org |access-date=16 April 2023}}</ref> can be used to look up the suitable soil pH range of a wide range of plants. Documents like ''Ellenberg's indicator values for British plants''<ref name="Hill1999">{{cite book |last1=Hill |first1=Mark Oliver |last2=Mountford |first2=J. Owen |last3=Roy |first3=David B. |last4=Bunce |first4=Robert G.H. |title=Ellenberg's indicator values for British plants. ECOFACT Volume 2. Technical Annex |date=1999 |publisher=[[UK Centre for Ecology & Hydrology|Institute of Terrestrial Ecology]] |location=Huntingdon, United Kingdom |isbn=978-1-870393-48-5 |url=https://nora.nerc.ac.uk/id/eprint/6411/ |access-date=16 April 2023}}</ref> can also be consulted.

However, a plant may be intolerant of a particular pH in some soils as a result of a particular mechanism, and that mechanism may not apply in other soils. For example, a soil low in [[molybdenum]] may not be suitable for [[soybean]] plants at pH 5.5, but soils with sufficient molybdenum allow optimal growth at that pH.<ref name="Sumner2002"/> Similarly, some calcifuges (plants intolerant of high-pH soils) can tolerate calcareous soils if sufficient phosphorus is supplied.<ref name="Lee1998">{{cite journal |last1=Lee |first1=John A. |title=The calcicole-calcifuge problem revisited |journal=Advances in Botanical Research |date=1998 |volume=29 |page=13 |url=https://books.google.com/books?id=1YigNjS78xsC&pg=PA13 |access-date=23 April 2023 |isbn=978-0-08-056183-7}}</ref> Another confounding factor is that different varieties of the same species often have different suitable soil pH ranges. Plant breeders can use this to breed varieties that can tolerate conditions that are otherwise considered unsuitable for that species – examples are projects to breed aluminium-tolerant and manganese-tolerant varieties of cereal crops for food production in strongly acidic soils.<ref name="Scott1989">{{cite book |last1=Scott |first1=Brendan J. |last2=Fisher |first2=J.A. |editor1-last=Robson |editor1-first=Alan D.|title=Soil acidity and plant growth |date=1989 |publisher=[[Academic Press]] |location=Sydney, Australia |isbn=978-0-12-590655-5 |pages=167–203 |url=https://lib-nhwjx7zkiyjoeipfyn7m37ed.late.re/book/2218580/813186 |access-date=23 April 2023 |chapter=Selection of genotypes tolerant of aluminium and manganese |doi=10.1016/B978-0-12-590655-5.50010-4}}</ref>

The table below gives suitable soil pH ranges for some widely cultivated plants as found in the ''USDA PLANTS Database''.<ref name="USDA2017"/> Some species (like ''[[Pinus radiata]]'' and ''[[Opuntia ficus-indica]]'') tolerate only a narrow range in soil pH, whereas others (such as ''[[Vetiveria zizanioides]]'') tolerate a very wide pH range.

{| class="wikitable sortable"
! rowspan=2|Scientific name !!  rowspan=2|Common name !! colspan=2|pH range
|-
!  Minimum !! Maximum
|-
| ''[[Chrysopogon zizanioides]]'' || vetiver grass || 3.0 || 8.0
|-
|''[[Pinus rigida]]''|| pitch pine || 3.5 || 5.1
|-
|''[[Rubus chamaemorus]]''|| cloudberry || 4.0 || 5.2
|-
|''[[Ananas comosus]]''|| pineapple|| 4.0|| 6.0
|-
|''[[Coffea arabica]]''|| Arabian coffee || 4.0|| 7.5
|-
|''[[Rhododendron arborescens]]''|| smooth azalea|| 4.2 || 5.7
|-
| ''[[Pinus radiata]]''|| Monterey pine|| 4.5|| 5.2
|-
| ''[[Carya illinoinensis]]''|| pecan|| 4.5|| 7.5
|-
| ''[[Tamarindus indica]]''|| tamarind|| 4.5|| 8.0
|-
| ''[[Vaccinium corymbosum]]''|| highbush blueberry|| 4.7|| 7.5
|-
| ''[[Manihot esculenta]]''|| cassava|| 5.0|| 5.5
|-
| ''[[Morus alba]]''||white mulberry||5.0||7.0
|-
| ''[[Malus]]''||apple||5.0||7.5
|-
| ''[[Pinus sylvestris]]''||Scots pine||5.0||7.5
|-
| ''[[Carica papaya]]''||papaya||5.0||8.0
|-
| ''[[Cajanus cajan]]''||pigeonpea||5.0||8.3
|-
| ''[[Pyrus communis]]''||common pear||5.2||6.7
|-
| ''[[Solanum lycopersicum]]''||garden tomato||5.5||7.0
|-
| ''[[Psidium guajava]]''||guava||5.5||7.0
|-
| ''[[Nerium oleander]]''||oleander||5.5||7.8
|-
| ''[[Punica granatum]]''||pomegranate||6.0||6.9
|-
| ''[[Viola sororia]]''||common blue violet||6.0||7.8
|-
| ''[[Caragana arborescens]]''||Siberian peashrub||6.0||9.0
|-
| ''[[Cotoneaster integerrimus]]''||cotoneaster||6.8||8.7
|-
| ''[[Opuntia ficus-indica]]''||Barbary fig (pricklypear)||7.0||8.5
|}

In natural or near-natural [[Plant community|plant communities]], the various pH preferences of plant species (or [[Ecotype|ecotypes]]) at least partly determine the composition and [[biodiversity]] of vegetation. While both very low and very high pH values are detrimental to plant growth, there is an increasing trend of plant biodiversity along the range from extremely acidic (pH 3.5) to strongly alkaline (pH 9) soils, i.e. there are more [[calcicole]] than [[calcifuge]] species, at least in terrestrial environments.<ref>{{cite journal |last1=Chytrý |first1=Milan |last2=Tichý |first2=Lubomír |last3=Rolček |first3=Jan |title=Local and regional patterns of species richness in Central European vegetation types along the pH/calcium gradient |journal=Folia Geobotanica |date=December 2003 |volume=38 |issue=4 |pages=429–42 |doi=10.1007/BF02803250 |bibcode=2003FolGe..38..429C |s2cid=13016841 |url=https://www.researchgate.net/publication/225452386 |access-date=7 May 2023}}</ref><ref>{{cite journal |last1=Pärtel |first1=Meelis |last2=Helm |first2=Aveliina |last3=Ingerpuu |first3=Nele |last4=Reier |first4=Ülle |last5=Tuvi |first5=Eva-Liis |title=Conservation of Northern European plant diversity: the correspondence with soil pH |journal=[[Biological Conservation (journal)|Biological Conservation]] |date=December 2004 |volume=120 |issue=4 |pages=525–31 |doi=10.1016/j.biocon.2004.03.025 |bibcode=2004BCons.120..525P |url=https://www.academia.edu/3276150 |access-date=7 May 2023}}</ref> Although widely reported and supported by experimental results,<ref>{{cite journal |last1=Crawley |first1=Michael J. |last2=Johnston |first2=A. Edward |last3=Silvertown |first3=Jonathan |last4=Dodd |first4=Mike |last5=de Mazancourt |first5=Claire |last6=Heard |first6=Matthew S. |last7=Henman |first7=D. F. |last8=Edwards |first8=Grant R. |title=Determinants of species richness in the Park Grass Experiment |journal=[[The American Naturalist|American Naturalist]] |date=February 2005 |volume=165 |issue=2 |pages=179–92 |doi=10.1086/427270 |pmid=15729649 |bibcode=2005ANat..165..179C |s2cid=7389457 |url=https://www.researchgate.net/publication/8003273 |access-date=14 May 2023}}</ref><ref>{{cite journal |last1=Poozesh |first1=Vahid |last2=Castillon |first2=Pierre |last3=Cruz |first3=Pablo |last4=Bertoni |first4=Georges |title=Re-evaluation of the liming-fertilization interaction in grasslands on poor and acid soils |journal=Grass and Forage Science |date=June 2010 |volume=65 |issue=2 |pages=260–72 |doi=10.1111/j.1365-2494.2010.00744.x |bibcode=2010GForS..65..260P |url=https://www.academia.edu/62428696 |access-date=14 May 2023}}</ref> the observed increase of plant species richness with pH is still in need of a clearcut explanation. [[Competitive exclusion principle|Competitive exclusion]] between plant species with overlapping pH ranges most probably contributes to the observed shifts of vegetation composition along pH gradients.<ref>{{cite journal |last1=Prince |first1=Candice M. |last2=MacDonald |first2=Gregory E. |last3=Ferrell |first3=Jason A. |last4=Sellers |first4=Brent A. |last5=Wang |first5=Jingjing |title=Impact of soil pH on cogongrass (Imperata cylindrica) and bahiagrass (Paspalum notatum) competition |journal=[[Weed Technology]] |date=2018 |volume=32 |issue=3 |pages=336–41 |doi=10.1017/wet.2018.3 |bibcode=2018WeedT..32..336P |s2cid=91112353 |url=https://bioone.org/journals/weed-technology/volume-32/issue-3/wet.2018.3/Impact-of-Soil-pH-on-Cogongrass-Imperata-cylindrica-and-Bahiagrass/10.1017/wet.2018.3.short|url-access=subscription }}</ref>

==pH effects on soil biota==
[[Soil biota]] (soil [[Microbiota|microflora]], soil animals) are sensitive to soil pH, either directly upon contact or after soil ingestion or indirectly through the various soil properties to which pH contributes (e.g. [[nutrient]] status, [[metal toxicity]], [[humus form]]). According to the various physiological and behavioural [[Adaptation|adaptations]] of soil biota, the species composition of soil microbial and animal communities varies with soil pH.<ref>{{cite journal |last1=Lauber |first1=Christian L. |last2=Hamady |first2=Micah |last3=Knight |first3=Rob |last4=Fierer |first4=Noah |title=Pyrosequencing-based assessment of soil pH as a predictor of soil bacterial community structure at the continental scale |journal=[[Applied and Environmental Microbiology]] |date=August 2009 |volume=75 |issue=15 |pages=5111–20 |doi=10.1128/AEM.00335-09 |pmid=19502440 |pmc=2725504 |bibcode=2009ApEnM..75.5111L }}</ref><ref name="Loranger2001"> {{cite journal |last1=Loranger |first1=Gladys |last2=Bandyopadhyaya |first2=Ipsa |last3=Razaka |first3=Barbara |last4=Ponge |first4=Jean-François |title=Does soil acidity explain altitudinal sequences in collembolan communities? |journal=[[Soil Biology and Biochemistry]] |date=March 2001 |volume=33 |issue=3 |pages=381–93 |doi=10.1016/S0038-0717(00)00153-X |bibcode=2001SBiBi..33..381L |s2cid=84523833 |url=https://www.academia.edu/48908569 |access-date=4 June 2023}}</ref> Along altitudinal gradients, changes in the species distribution of soil animal and microbial communities can be at least partly ascribed to variation in soil pH.<ref name="Loranger2001"/><ref>{{cite journal |last1=Tian |first1=Qiuxiang |last2=Jiang |first2=Ying |last3=Tang |first3=Yanan |last4=Wu |first4=Yu |last5=Tang |first5=Zhiyao |last6=Liu |first6=Feng |title=Soil pH and organic carbon properties drive soil bacterial communities in surface and deep layers along an elevational gradient |journal=[[Frontiers in Microbiology]] |date=30 July 2021 |volume=12 |article-number=646124 |doi=10.3389/fmicb.2021.646124 |pmid=34394018 |pmc=8363232 |doi-access=free}}</ref> The shift from toxic to non-toxic forms of [[aluminium]] around pH5 marks the passage from acid-tolerance to acid-intolerance, with few changes in the species composition of soil communities above this threshold, even in [[calcareous soils]].<ref>{{cite journal |last=Ponge |first=Jean-François |title=Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems |journal=Pedobiologia |date=July 1993 |volume=37 |issue=4 |pages=223–44 |doi=10.1016/S0031-4056(24)00100-8 |bibcode=1993Pedob..37..223P |url=https://www.academia.edu/50930736 |access-date=11 June 2023}}</ref><ref>{{cite journal |last1=Desie |first1=Ellen |last2=Van Meerbeek |first2=Koenraad |last3=De Wandeler |first3=Hans |last4=Bruelheide |first4=Helge |last5=Domisch |first5=Timo |last6=Jaroszewicz |first6=Bogdan |last7=Joly |first7=François-Xavier |last8=Vancampenhout |first8=Karen |last9=Vesterdal |first9=Lars |last10=Muys |first10=Bart |title=Positive feedback loop between earthworms, humus form and soil pH reinforces earthworm abundance in European forests |journal=[[Functional Ecology (journal)|Functional Ecology]] |date=August 2020 |volume=34 |issue=12 |pages=2598–2610 |doi=10.1111/1365-2435.13668 |bibcode=2020FuEco..34.2598D |s2cid=225182565 |url=https://besjournals.onlinelibrary.wiley.com/doi/pdfdirect/10.1111/1365-2435.13668 |access-date=11 June 2023|hdl=1893/31777 |hdl-access=free |url-access=subscription }}</ref> Soil animals exhibit distinct pH preferences when allowed to exert a choice along a range of pH values,<ref>{{cite journal |last1=Van Straalen |first1=Nico M. |last2=Verhoef |first2=Herman A. |title=The development of a bioindicator system for soil acidity based on arthropod pH preferences |journal=[[Journal of Applied Ecology]] |date=February 1997 |volume=34 |issue=1 |pages=217–32 |doi=10.2307/2404860 |jstor=2404860 |bibcode=1997JApEc..34..217V }}</ref> explaining that various field distributions of soil organisms, motile microbes included, could at least partly result from active movement along pH gradients.<ref>{{cite journal |last1=He |first1=Bin |last2=Wang |first2=Zhipeng |last3=Xu |first3=Changhui |last4=Shen |first4=Runjie |last5=Hu |first5=Sanqing |title=The study on pH gradient control in solution for driving bacteria |journal=Biocybernetics and Biomedical Engineering |date=2013 |volume=33 |issue=2 |pages=88–95 |doi=10.1016/j.bbe.2013.03.003 |url=https://daneshyari.com/article/preview/5279.pdf |access-date=18 June 2023}}</ref><ref>{{cite journal |last1=Wang |first1=Congli |last2=Bruening |first2=George |last3=Williamson |first3=Valerie M. |title=Determination of preferred pH for root-knot nematode aggregation using pluronic F-127 gel |journal=[[Journal of Chemical Ecology]] |date=20 October 2009 |volume=35 |issue=10 |pages=1242–51 |doi=10.1007/s10886-009-9703-8 |pmid=19838866 |pmc=2780626 |bibcode=2009JCEco..35.1242W |s2cid=8403899 |url=https://link.springer.com/content/pdf/10.1007/s10886-009-9703-8.pdf |access-date=18 June 2023}}</ref> Like for plants, [[Competition (biology)|competition]] between acido-tolerant and acido-intolerant soil-dwelling organisms was suspected to play a role in the shifts in species composition observed along pH ranges.<ref>{{cite journal |last=Hågvar |first=Sigmund |title=Reactions to soil acidification in microarthropods: is competition a key factor? |journal=Biology and Fertility of Soils |date=April 1990 |volume=9 |issue=2 |pages=178–81 |doi=10.1007/BF00335804 |bibcode=1990BioFS...9..178H |s2cid=5099516 |url=https://link.springer.com/content/pdf/10.1007/BF00335804.pdf |access-date=25 June 2023}}</ref>

The opposition between acido-tolerance and acido-intolerance is commonly observed at [[species]] level within a [[genus]] or at genus level within a [[Family (biology)|family]], but it also occurs at much higher [[taxonomic rank]], like between soil fungi and bacteria, here too with a strong involvement of competition.<ref>{{cite journal |last1=Rousk |first1=Johannes |last2=Brookes |first2=Philip C. |last3=Bååth |first3=Erland |title=Investigating the mechanisms for the opposing pH relationships of fungal and bacterial growth in soil |journal=[[Soil Biology and Biochemistry]] |date=June 2010 |volume=42 |issue=6 |pages=926–34 |doi=10.1016/j.soilbio.2010.02.009 |bibcode=2010SBiBi..42..926R |url=https://repository.rothamsted.ac.uk/download/8a1be26f776ec3bb7631f8512543ad4ea02e370f15d38ee5f89db2178ff4cdba/569077/1-s2.0-S0038071710000623-main.pdf |access-date=25 June 2023}}</ref>
It has been suggested that soil organisms more tolerant of soil acidity, and thus living mainly in soils at pH less than 5, were more [[Primitive (phylogenetics)|primitive]] than those intolerant of soil acidity.<ref>{{cite journal |last=Ponge |first=Jean-François |title=Acidophilic Collembola: living fossils? |journal=Contributions from the Biological Laboratory, Kyoto University |date=March 2000 |volume=29 |pages=65–74 |url=https://www.researchgate.net/publication/45794239 |access-date=2 July 2023}}</ref> A [[Cladistics|cladistic]] analysis on the [[Springtail|collembolan]] genus [[Willemia]] showed that tolerance to soil acidity was correlated with tolerance of other [[Stress (biology)|stress]] factors and that stress tolerance was an [[Ancestral reconstruction|ancestral]] character in this genus.<ref>{{cite journal |last1=Prinzing |first1=Andreas |last2=D'Haese |first2=Cyrille A. |last3=Pavoine |first3=Sandrine |last4=Ponge |first4=Jean-François |title=Species living in harsh environments have low clade rank and are localized on former Laurasian continents: a case study of Willemia (Collembola) |journal=[[Journal of Biogeography]] |date=February 2014 |volume=41 |issue=2 |pages=353–65 |doi=10.1111/jbi.12188 |bibcode=2014JBiog..41..353P |s2cid=86619537 |url=https://www.researchgate.net/publication/259967386 |access-date=2 July 2023}}</ref> However the generality of these findings remains to be established.

At low pH,  the [[oxidative stress]] induced by aluminium (Al<sup>3+</sup>) affects soil animals the body of which is not protected by a thick [[chitinous]] [[exoskeleton]] like in [[Arthropod|arthropods]], and thus are in more direct contact with the soil solution, e.g. [[Protist|protists]], [[Nematode|nematodes]], [[Rotifer|rotifers]] ([[microfauna]]), [[Enchytraeidae|enchytraeids]] ([[mesofauna]]) and [[Earthworm|earthworms]] ([[macrofauna]]).<ref>{{cite journal |last1=Li |first1=Yin-Sheng |last2=Sun |first2=Jing |last3=Robin |first3=Paul |last4=Cluzeau |first4=Daniel |last5=Qiu |first5=Jiangping |title=Responses of the earthworm ''Eisenia andrei'' exposed to sublethal aluminium levels in an artificial soil substrate |journal=Chemistry and Ecology |date=April 2014 |volume=30 |issue=7 |pages=611–21 |doi=10.1080/02757540.2014.881804 |bibcode=2014ChEco..30..611L |s2cid=97315212 |url=https://www.researchgate.net/publication/267103805 |access-date=2 July 2023}}</ref>

Effects of pH on soil biota can be mediated by the various functional interactions of soil [[Food web|foodwebs]]. It has been shown experimentally that the collembolan ''[[Heteromurus nitidus]]'', commonly living in soils at pH higher than 5, could be cultured in more acid soils provided that predators were absent.<ref>{{cite journal |last1=Salmon |first1=Sandrine |last2=Ponge |first2=Jean-François |title=Distribution of ''Heteromurus nitidus'' (Hexapoda, Collembola) according to soil acidity: interactions with earthworms and predator pressure |journal=[[Soil Biology and Biochemistry]] |date=July 1999 |volume=31 |issue=8 |pages=1161–70 |doi=10.1016/S0038-0717(99)00034-6 |bibcode=1999SBiBi..31.1161S |url=https://www.researchgate.net/publication/222442654 |access-date=16 July 2023}}</ref> Its attraction to earthworm [[Excretion|excreta]] ([[mucus]], [[urine]], [[Feces|faeces]]), mediated by [[ammonia]] emission,<ref>{{cite journal |last=Salmon |first=Sandrine |title=Earthworm excreta (mucus and urine) affect the distribution of springtails in forest soils |journal=Biology and Fertility of Soils |date=November 2001 |volume=34 |issue=5 |pages=304–10 |doi=10.1007/s003740100407 |bibcode=2001BioFS..34..304S |s2cid=33455553 |url=https://www.researchgate.net/publication/225534906 |access-date=23 July 2023}}</ref> provides food and shelter within earthworm burrows in mull [[humus form]]s associated with less acid soils.<ref>{{cite journal |last=Salmon |first=Sandrine |title=The impact of earthworms on the abundance of Collembola: improvement of food resources or of habitat? |journal=Biology and Fertility of Soils |date=17 September 2004 |volume=40 |issue=5 |pages=523–33 |doi=10.1007/s00374-004-0782-y |bibcode=2004BioFS..40..323S |s2cid=9671870 |url=https://www.researchgate.net/publication/225542635 |access-date=23 July 2023}}</ref>

==Effects of soil biota on soil pH==
Soil biota affect soil pH directly through [[excretion]], and indirectly by acting on the physical environment. Many soil fungi, although not all of them, acidify the soil by excreting [[oxalic acid]], a product of their [[Respiratory system|respiratory]] metabolism. Oxalic acid [[Precipitation (chemistry)|precipitates]] calcium, forming insoluble [[Crystal|crystals]] of [[calcium oxalate]] and thus depriving the soil solution from this necessary element.<ref>{{cite journal |last1=Casarin |first1=Valter |last2=Plassard |first2=Claude |last3=Souche |first3=Gérard |last4=Arvieu |first4=Jean-Claude |title=Quantification of oxalate ions and protons released by ectomycorrhizal fungi in rhizosphere soil |journal=Agronomie |date=July 2003 |volume=23 |issue=5–6 |pages=461–69 |doi=10.1051/agro:2003020 |bibcode=2003AgSD...23..461C |s2cid=84663116 |url=https://www.researchgate.net/publication/41706199 |access-date=20 August 2023}}</ref> On the opposite side, earthworms exert a [[buffer solution|buffering effect]] on soil pH through their excretion of [[mucus]], endowed with [[Amphoterism|amphoteric]] properties.<ref>{{cite journal |last=Schrader |first=Stefan |title=Influence of earthworms on the pH conditions of their environment by cutaneous mucus secretion |journal=[[Zoologischer Anzeiger]] |date=1994 |volume=233 |issue=5–6 |pages=211–19}}</ref>

By mixing organic matter with mineral matter, in particular [[clay]] particles, and by adding mucus as a glue for some of them, burrowing soil animals, e.g. [[fossorial]] [[Rodent|rodents]], [[Mole (animal)|mole]]s, [[Earthworm|earthworms]], [[Termite|termites]], some [[Millipede|millipedes]] and [[fly]] larvae, contribute to decrease the natural acidity of raw organic matter, as observed in [[mull humus]].<ref>{{cite journal |last1=Guhra |first1=Tom |last2=Stolze |first2=Katharina |last3=Schweizer |first3=Steffen |last4=Totsche |first4=Kai Uwe |title=Earthworm mucus contributes to the formation of organo-mineral associations in soil |journal=[[Soil Biology and Biochemistry]] |date=June 2020 |volume=145 |issue=107785 |pages=1–10 |article-number=107785 |doi=10.1016/j.soilbio.2020.107785 |bibcode=2020SBiBi.14507785G |url=https://www.researchgate.net/publication/339991303 |access-date=27 August 2023|doi-access=free |hdl=21.11116/0000-0006-600A-3 |hdl-access=free }}</ref><ref>{{cite journal |last1=Zanella |first1=Augusto |last2=Ponge |first2=Jean-François |last3=Briones |first3=Maria J. I. |title=Humusica 1, article 8: Terrestrial humus systems and forms – Biologicalactivity and soil aggregates, space-time dynamics
 |journal=Applied Soil Ecology |date=January 2018 |volume=122 |issue=1 |pages=103–37 |doi=10.1016/j.apsoil.2017.07.020 |bibcode=2018AppSE.122..103Z |url=https://www.academia.edu/95860148 |access-date=27 August 2023|hdl=11577/3257337 |hdl-access=free }}</ref>

==Changing soil pH==

===Increasing pH of acidic soil===
Finely ground [[agricultural lime]] is often applied to acid soils to increase soil pH ([[Liming (soil)|liming]]). The amount of [[limestone]] or [[chalk]] needed to change pH is determined by the [[Mesh (scale)|mesh]] size of the lime (how finely it is ground) and the [[Soil#Buffering|buffering capacity]] of the soil. A high mesh size (60 mesh = 0.25&nbsp;mm; 100 mesh = 0.149&nbsp;mm) indicates a finely ground lime that will react quickly with soil acidity. The buffering capacity of a soil depends on the clay content of the soil, the type of clay, and the amount of organic matter present, and may be related to the soil [[cation exchange capacity]].  Soils with high clay content will have a higher buffering capacity than soils with little clay, and soils with high organic matter will have a higher buffering capacity than those with low organic matter.<ref>{{cite journal |last1=Minhal |first1=Fibrianty |last2=Ma'as |first2=Aswar |last3=Hanudin |first3=Eko |last4=Sudira |first4=Putu |title=Improvement of the chemical properties and buffering capacity of coastal sandy soil as affected by clay and organic by-product application  |journal=Soil and Water Research |date=June 2020 |volume=15 |issue=2 |pages=93–100 |doi=10.17221/55/2019-SWR |bibcode=2020SWatR..15...93M |url=https://www.old-aj.cz/publicFiles/55_2019-SWR.pdf |access-date=3 September 2023}}</ref> Soils with higher buffering capacity require a greater amount of lime to achieve an equivalent change in pH.<ref name="Aitken1990">{{cite journal |last1=Aitken |first1=R.L. |last2=Moody |first2=Philip W. |last3=McKinley |first3=P.G. |title=Lime requirement of acidic Queensland soils. I. Relationships between soil properties and pH buffer capacity |journal=[[Australian Journal of Soil Research]] |date=1990 |volume=28 |issue=5 |pages=695–701 |doi=10.1071/SR9900695 |bibcode=1990SoilR..28..695A |url=https://www.publish.csiro.au/SR/SR9900695|url-access=subscription }}</ref> The buffering of soil pH is often directly related to the quantity of aluminium in soil solution and taking up exchange sites as part of the [[Cation-exchange capacity|cation exchange capacity]].  This aluminium can be measured in a soil test in which it is extracted from the soil with a salt solution, and then is quantified with a laboratory analysis.  Then, using the initial soil pH and the aluminium content, the amount of lime needed to raise the pH to a desired level can be calculated.<ref>{{cite journal |last=Bartlett |first=Richmond |date=1982 |title=Reactive aluminum in the Vermont Soil Test |journal=Communications in Soil Science and Plant Analysis |volume=13 |issue=7| pages=497–506 |doi=10.1080/00103628209367289 |bibcode=1982CSSPA..13..497B |url=https://www.tandfonline.com/doi/epdf/10.1080/00103628209367289|url-access=subscription }}</ref>

Amendments other than agricultural lime that can be used to increase the pH of soil include [[wood ash]], industrial calcium oxide ([[burnt lime]]), [[magnesium oxide]], [[basic slag]] ([[calcium silicate]]), and [[oyster]] shells.  These products increase the pH of soils through various [[acid–base reaction]]s. [[Calcium silicate]] neutralizes active acidity in the soil by reacting with H<sup>+</sup>&nbsp;ions to form [[Silicic acid|monosilicic acid]] (H<sub>4</sub>SiO<sub>4</sub>), a neutral solute.<ref name="VonUexkull1986">{{cite book |last1=von Uexkull |first1=H.R. |location=Rome, Italy |title=Efficient fertilizer use in acid upland soils of the humid tropics |date=1986 |publisher=[[Food and Agriculture Organization]] |isbn=978-92-5-102387-7 |pages=16–22 |url=https://edepot.wur.nl/494356 |access-date=10 September 2023 |language=en |chapter=Lime and liming}}</ref>

===Decreasing the pH of alkaline soil===
The pH of an alkaline soil can be reduced by adding acidifying agents or acidic organic materials. Elemental [[sulfur]] (90–99% S) has been used at application rates of {{convert|300|-|500|kg/ha|abbr=on}} – it slowly oxidises in the soil to form [[sulfuric acid]]. Acidifying fertilizers, such as [[ammonium sulfate]], [[ammonium nitrate]] and [[urea]], can help to reduce the pH of soil because ammonium oxidises to form [[nitric acid]]. Acidifying organic materials include [[peat]] or [[sphagnum]] peat moss.<ref name="Cox2010">{{cite web |title=Solutions to soil problems. II. High pH (alkaline soil) |url=https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1954&context=extension_curall |last1=Cox |first1=Loralie |last2=Koenig |first2=Rich |year=2010 |publisher=[[Utah State University]] |location=Logan, Utah |access-date=10 September 2023}}</ref>

However, in high-pH soils with a high calcium carbonate content (more than 2%), attempting to reduce the pH with acids can be very costly and ineffective. In such cases, it is often more efficient to add phosphorus, iron, manganese, copper, or zinc instead because deficiencies of these nutrients are the most common reasons for poor plant growth in calcareous soils.<ref>{{cite web |url=https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5293776.pdf |archive-url=https://web.archive.org/web/20170504044652/https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5293776.pdf |archive-date=May 4, 2017 |year=1998 |title=Soil quality indicators: pH |website=[[United States Department of Agriculture|USDA]], [[Natural Resources Conservation Service]] |access-date=10 September 2023}}</ref><ref name="Cox2010"/>

== See also ==
* [[Acid mine drainage]]
* [[Acid sulfate soil]]
* [[Cation-exchange capacity]]
* [[Fertilizer]]
* [[Liming (soil)]]
* [[Organic horticulture]]
*[[Redox gradient]]

==References==
{{Reflist}}

==External links==
*[https://journals.lww.com/soilsci/Citation/1965/07000/A_Study_of_the_Lime_Potential__5__Significance_of.3.aspx "A Study of Lime Potential, R.C. Turner, Research Branch, Canadian Department of Agriculture, 1965"]

{{soil science topics}}{{Plant nutrition}}{{Authority control}}

[[Category:Horticulture]]
[[Category:Organic gardening]]
[[Category:Plant nutrition]]
[[Category:Soil chemistry|PH]]