Cation exchange capacity (CEC) is important because it is a measure of the nutrient holding capacity of a soil. I was reminded this week of something that turfgrass managers should know about CEC in sand rootzones. The 2009 International Annual Meetings of the Agronomy, Crops, and Soil Science Societies of America were held this week at Pittsburgh. I was co-author on a paper presented there by Dr. Quirine Ketterings of Cornell University. The paper, entitled A Simple Method for Estimating Cation Exchange Capacity Across a Wide Range of Soils, showed the results of experiments on estimating CEC on a range of fifty different soils from around New York state.

What is important to know?

• Normal soil test procedures and the CEC values that you see on your soil reports are overestimated, especially for sand rootzones
• Cation ratios or percentages based on these erroneous CEC values are meaningless
• Addition of calcium, magnesium, and potassium fertilizers should be based on the amount of that element in the soil, and not on the percentage of that element
• A nearly foolproof method of applying the right amount of nutrients to turfgrass, even without having any idea of the soil CEC, is to base the nitrogen application rate on the turfgrass growth potential (varies with turf species and temperature) and then to apply an 8:1:4 ratio of nitrogen, phosphorus, and potassium

For more details and a technique to predict the actual CEC of your soils (because the value on your soil report is almost certainly wrong), read on. Take this quote from the abstract of the paper presented this week: The CEC_sum estimates based on Mehlich 3 or Morgan greatly overestimated the CEC of the fifty soils tested in this experiment. This is exactly what I found in my research on soil testing during my time at Cornell. The CEC estimates that you see on a soil report from your laboratory are almost certain to be very different from the actual CEC of the soil. And these errors in CEC as shown on soil reports are especially evident when the tested soils come from golf course putting greens or other sandy sites.

In my research (you can download the chapter about CEC from my dissertation here) I looked at 54 sand and soil samples from around the world. Many samples were collected from golf course sites in Asia. We found that the best way to estimate CEC in sand rootzones is to use a 0.01 M solution of SrCl₂ in a rapid five minute extraction. The standard soil tests such as Mehlich 3 produced an inflated value of the CEC. Because commercial soil testing laboratories do not use the SrCl₂ method, the best way to estimate CEC for the average turfgrass manager is to use an equation with only two variables (soil pH and soil organic matter) to predict the CEC. Surprising, for sand rootzones, this equation predicts CEC more accurately than does a conventional soil test.

CEC (mmol_c kg^-1) = soil organic matter/100 * (-311 + (268 * soil pH))

If you are more comfortable in CEC units of cmol_c kg^-1 (or meq/100 g), then divide the soil organic matter by 1000 rather than 100 as is shown in the equation above.