MSU Extension Soil Web pages for Consumers

What does CEC mean?  The Cation Exchange Capacity is a measure of the soil's ability to hold certain nutrients known as cations. The CEC on the soil test report is a calculation of the exchangeable calcium, magnesium, potassium, and hydrogen measured by the soil tests. This calculation is given in milli-equivalents (me.) per 100 grams of soil. The CEC value helps technicians make the potassium and magnesium interpretations and estimate the soil texture. CEC is related to the amount of fine organic matter and the amount of clay in soils. Very sandy soils, low in organic matter, commonly have CEC’s less than 5. A CEC between 10 and 15 is typical and usually adequate. 

Can CEC be improved? The CEC of a soil can be increased somewhat by increasing the soil’s organic matter content.

CEC and adding fertilizer and lime Soils containing a high percentage of organic matter also tend to have high cation exchange capacities. Sandy soils containing a low percentage of clay and organic matter have low exchange capacities. This explains why coarse textured (sandy) soils require more frequent applications of lime and fertilizer than soils containing more clay and organic matter.

% base saturation The ease with which a plant gains access to these nutrients depends somewhat on the relative percentages of the adsorbed cations. For this reason it is suggested that percentage saturation levels be held within loosely defined ranges. For example, a soil with base saturations of Calcium 70%, Magnesium 12% and Potassium 4%  is considered balanced for most crops and has a soil pH of about 6.5.

Clay and organic matter (humus) keep nutrients from washing out of soil.  If a soil has a low CEC and high sodium levels, up to half the cations in the soil may be in the water around the soil particles, and not actually held by the particles. These cations are very susceptible to being leached or drained away in the soil water.

Soils with a high CEC have a much lower percentage of cations in the soil water, so they are far less susceptible to nutrient loss by leaching.

Cations have charges The clay and organic components of the soil have a negative charge. As a result of these charges, positively charged ions (cations) such as hydrogen H+, potassium K+, ammonium NH+4, calcium Ca2+, magnesium Mg2+, aluminum Al3+, etc. are attracted to the surfaces of clay and organic soil particles. They are held there and exchanged with other ions in the solution or with ions at the plant root's surface.