Editing Soil (section)

====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>