Cation-exchange capacity
Cation-exchange capacity (CEC) is a measure of how many cations can be retained on soil particle surfaces.[1] Negative charges on the surfaces of soil particles bind positively-charged atoms or molecules (cations), but allow these to exchange with other positively charged particles in the surrounding soil water.[2] This is one of the ways that solid materials in soil alter the chemistry of the soil. CEC affects many aspects of soil chemistry, and is used as a measure of soil fertility, as it indicates the capacity of the soil to retain several nutrients (e.g. K+, NH4+, Ca2+) in plant-available form. It also indicates the capacity to retain pollutant cations (e.g. Pb2+).
Base saturation[edit]
Base saturation expresses the percentage of potential CEC occupied by the cations Ca2+, Mg2+, K+ or Na+.[1][4] These are traditionally termed "base cations" because they are non-acidic, although they are not bases in the usual chemical sense.[1] Base saturation provides an index of soil weathering[4] and reflects the availability of exchangeable cationic nutrients to plants.[1]
Anion-exchange capacity[edit]
Positive charges of soil minerals can retain anions by the same principle as cation exchange. The surfaces of kaolinite, allophane and iron and aluminium oxides often carry positive charges.[1] In most soils the cation-exchange capacity is much greater than the anion-exchange capacity, but the opposite can occur in highly weathered soils,[1] such as ferralsols (oxisols).