Editing Soil (section)

===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>