Knowra Cation-Exchange Capacity Cation-Exchange Capacity Cation-exchange capacity is the quantity of negatively charged sites in soil that can retain and exchange positively charged ions. It is commonly expressed as centimoles of charge per kilogram of soil.
Soil colloid : A tiny soil particle, such as clay or humus, with a large surface area and reactive electrical charges. Clay and organic colloids supply many of the charged sites counted in CEC.
Clay mineral : A fine-grained mineral formed by weathering or alteration, with layered structures and reactive surfaces. Clay mineral type and abundance strongly shape a soil’s CEC.
Soil fertility : The capacity of soil to support plant growth by supplying suitable nutrients, water, and physical conditions. CEC helps explain how well soil retains nutrient cations for plant uptake.
Effective cation-exchange capacity : The exchange capacity measured at a soil’s existing pH, including exchangeable aluminum where relevant. Unlike potential CEC measured at a set pH, it reflects current soil conditions.
Permanent charge : An electrical charge on a mineral surface that persists across ordinary changes in soil pH. Substitutions within clay minerals create CEC sites that do not depend on pH.
Humus : The stable, decomposed organic matter in soil, formed from the remains of plants and other organisms. Humus commonly carries many pH-dependent exchange sites.
Nutrient leaching : The downward movement of dissolved nutrients through soil with percolating water. Low CEC can leave nutrient cations more vulnerable to removal in drainage water.
Potential cation-exchange capacity : Cation-exchange capacity measured after adjusting soil to a specified pH, commonly near neutral. Its controlled-pH value can exceed the effective capacity of acidic variable-charge soils.
Variable charge : A surface charge that changes with pH as chemical groups gain or lose protons. These sites make CEC rise or fall with soil pH.
Cation : An atom or molecule with a net positive electrical charge. CEC measures soil’s capacity to retain and exchange these ions.
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