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Chesley, Ontario | You and a lot of soil scientists, John! The difference between the soil scientists, and the ones who use CEC most frequently, is that the scientists KNOW they don't fully understand it.
Despite that, I'll try to give a 50 cent explanation of the concept. CEC stands for Cation Exchange Capacity, and a Cation is a positively charged ion when in solution - Ca, Mg, NH4, K, Na, Al, Fe, H, etc. The clay and organic matter in the soil have negative charges, so the positively charged cations stick to these negative sites. CEC is simply the amount of negative charge in the soil...the more negative charge, the higher the CEC and the more cations the soil will hold on to.
The accurate measure of CEC is to saturate the soil with a cation like barium, wash out any excess, and then analyze the soil to see how much barium is held there. This is messy and time consuming, so practically all the reported CECs on a soil test report are calculated by adding up the cations that have been extracted. This shortcut works reasonably well in acid soils, but it starts to fall apart as the pH rises above 7. In alkaline soils, there are other places for calcium and magnesium to hang out than just on the cation exchange sites, but a lot of this extra Ca and Mg is dissolved in the soil test extractant. When the cations are added up, the amount of Ca and Mg is inflated, and so is the total CEC.
You are probably better off using the rule of thumb that the CEC of the soil is about half the clay percentage of the soil, plus two times the percent organic matter. (e.g. a silt loam soil with 12% clay and 4% OM would have a CEC of about (12/2) + (4*2) = 6 + 8 = 14) This is probably closer than the calculated number on the soil test report.
In some areas, the CEC affects the optimum amount of potash needed by the crop, and this is reflected in the recommendations by the land grant colleges. In the soil types found in Ontario, we have not found any relationship between CEC and optimum K rates, so it is not included in our fertilizre recommendations as a factor. | |
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Cec of soil
