Glossary:Exchangeable cation percentage (ECP)

The extent to which the adsorption complex of a soil is occupied by a particular cation. The proportion (in percent) of the total cation exchange capacity of a soil (at a given pH) which is satisfied by a given species of cation at a given point in time. The exchangeable cations are determined by displacing them with a concentrated solution consisting of a different type of cation. The type and quantity of displaced cations are collected and measured in the laboratory. The resulting data give both the cation exchange capacity and the relative cation percentages. For example, if 8 meq calcium, 4 meq potassium, 4 meq sodium, 1 meq aluminum, 1 meq magnesium, 1 meq ammonium, and 1 meq of other miscellaneous cations were displaced by cation exchange from 100 grams of soil, then the cation exchange capacity would be 20 milliequivalents per 100 grams of soil, and relative exchange able cation percentages would be 40, 20, 20, 5, 5, 5, and 5, respectively. It is expressed as follows