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Corn+Soybean Digest

What is Cation Exchange Capacity?

Growers and ag professionals frequently ask questions about interpreting the results of soil sample analysis, and about the importance of the measurement called cation exchange capacity (CEC). I'll attempt to explain what this means.

CEC is a measure of the capacity of soils to retain positively charged nutrients. In soils, there are negative charges associated with clay-sized particles and soil organic matter.

There is a basic chemistry concept that opposite electrical charges are attracted to each other. In soils, potassium (K+), calcium (Ca++), magnesium (Mg++), hydrogen (H+) and ammonium (+) have positive charges. Therefore, these ions are attracted to the negative electrical charges on clay and organic matter and do not move through the soil profile.

On the other hand, nitrate and sulfate have negative electrical charges, are not attracted to the negative electrical charges and move through the soil.

In a laboratory, the number of electrical charges can be measured and is reported in units called millie-equivalents per 100 grams. This is not a particularly important piece of information, however.

WHAT DOES THE CEC of soils tell us? Nothing, other than soil texture. For example, sandy soils have a CEC of 5-7. Soils with a silt loam texture have a CEC of 10-12. Soils with higher clay content have a higher CEC. These values change slightly with organic matter content - but not much.

A measure of CEC has no relationship to fertilizer recommendations in the upper Midwest. Recommendations of phosphate, potash, zinc and other nutrients are based on the relative supply of the various nutrients in soils. This relative supply is not affected by soil texture or CEC. So, even though a grower may get this information on a soil test report, the CEC can be ignored unless there is some reason to know the texture of the soil.

Those who believe that CEC is important also believe in the concept of calcium to magnesium ratios. In their eyes, the ratio of calcium to magnesium associated with the negative charges should be 6.5 to 1. By this concept, a lower ratio means there is too much magnesium and they believe this is bad. Their solution is to add gypsum, and this is expensive without producing any return for the expense.

The concept of calcium to magnesium ratio has been the focus of considerable research in Ohio, Wisconsin and Minnesota. This research has shown, without question, that there is no relationship whatsoever between this ratio and fertilizer recommendations.

If someone is providing advice pertaining to fertilizer use that is based on CEC, that advice should be ignored. Fertilizer recommendations based on CEC lead to excessive fertilizer costs without an increase in yield.

Isn't it good to know that some information is not important and can be ignored?

Editor's Note: George Rehm is a nutrient management specialist and professor emeritus from the University of Minnesota.

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