Did you know that the cation exchange capacity (CEC) reported on standard soil tests is not really a measure of CEC? On most soil test reports, the CEC (if it is reported at all) is an estimate of CEC based on the extracted cations from the soil. The idea is that if cations such as calcium, magnesium, potassium, and sodium are extracted from the soil, a summation of those cations will be close to the CEC of the soil.

In my graduate school research, I measured the CEC of sand rootzones and compared the measured CEC to multiple methods of estimating CEC. It turns out that for sand rootzones, the standard soil test summation methods overestimate the CEC by a substantial amount. To get an accurate estimate of CEC in a sand rootzone, one doesn't need to extract the cations from the soil. That's because sands are inert. The sand itself has a CEC of 0. 

The CEC in a sand rootzone comes from the organic matter (OM), and the contribution of OM to CEC can be calculated from this equation:

CEC = (-311 + 268 x soil pH) x (soil OM % ÷ 100)

The CEC estimate using that equation will give the result in units of millimoles of negative charge in the soil per kg of soil (mmol_c/kg).

For more about this, see this poster from the 2005 Oregon State University turfgrass field day, or read the chapter on predicting cation exchange capacity from my dissertation.

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