Anion exchange capacity (AEC)

Anion exchange capacity should be thought of as the soil’s ability to remove anions from the soil water solution and sequester those for later exchange as the plant roots release carbonate anions to the solution. Those colloids which have low CEC tend to have some AEC. Amorphous and sesquioxide clays have the highest AEC, followed by the iron oxides. Levels of AEC are much lower than for CEC. Phosphates tend to be held at anion exchange sites.
Iron and aluminum hydroxide clays are able to exchange their hydroxide anions (OH-) for other anions. The order reflecting the strength of anion adhesion is as follows:
H2PO4- replaces SO42- replaces NO3- replaces Cl-
The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100 g dry soil. 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).

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