Cation-exchange Capacity - Calculation of CEC

Calculation of CEC

The CEC is the number of positive charges that a soil can contain. It is usually described as the amount of equivalents necessary to fill the soil capacity. In soil science, an equivalent is defined by the number of charges in terms of a given number of hydrogen ions. As hydrogen ions have only one positive charge (H+), this makes calculations relatively simple. An equivalent of Al3+ would have one third as many ions for the same amount of charges, and Ca2+ would have half as many ions.

Translation from meq/100g to an every day unit, like lb/acre of available nutrients, can be made via calculation, that considers the atomic weight, the ion's valence, and by estimating the soil depth and its density. Mengel gives the following correspondence for 1 meq/100g:

Calcium, 400 lb/acre

Magnesium, 240 lb/acre

Potassium, 780 lb/acre

Ammonium, 360 lb/acre