Explaining Cation-Exchange Capacity

By Charles Walters & C.J. Fenzau

The cation-exchange capacity measures the ability of the soil to exchange nutrients.

The first order of business for the soil colloid is to hold nutrients —nutrients that can be traded off as the roots of a plant demand them. Thus the first index from the laboratory — the energy of the clay and the humus.

Almost all laboratories report cation exchange capacity, and they do this in terms of milliequivalents, or ME. If it helps, you can think of an electrician mea­suring in terms of volts and amperes, or a physicist measuring magnetic energy in terms of ergs and joules. The soil laboratory has its own lexicon. It expresses col­loidal energy in terms of milliequivalents of a total exchange capacity, since soil colloids — composed of clay and organic matter — are negatively charged parti­cles. Negative attracts positive. Cation nutrients are attracted and held on the soil colloids. Since anions are not attracted by the negative soil colloids, they remain free to move in the soil solution or water.

ME represents the amount of colloidal energy need­ed to absorb and hold to the soil’s colloid in the top seven inches of one acre of soil 400 pounds of calcium, or 240 pounds of magnesium, or 780 pounds of potas­sium, or simply 20 pounds of exchangeable hydrogen.

Source: Eco-Farm: An Acres U.S.A. Primer (1979)