Organic matter
Cation-exchange capacity arises from various negative charges on soil particle surfaces, especially those of clay minerals and soil organic matter. Phyllosilicate clays consist of layered sheets of aluminium and silicon oxides. The replacement of aluminium or silicon atoms by other elements with lower charge (e.g. Al3+ replaced by Mg2+) can give the clay structure a net negative charge. This charge does not involve deprotonation and is therefore pH-independent, and called permanent charge. In addition, the edges of these sheets expose many acidic hydroxyl groups that are deprotonated to leave negative charges at the pH levels in many soils. Organic matter also makes a very significant contribution to cation exchange, due to its large number of charged functional groups. CEC is typically higher near the soil surface, where organic matter content is highest, and declines with depth. The CEC of organic matter is highly pH-dependent.