PRINCIPLE
The cation exchange capacity (CEC) of an acid soil can be determined by adding the milliequivalents of the determined bases (Ca, Mg, K, and Na where applicable) to the milliequivalents of exchangeable hydrogen, expressed as milliequivalents per 100 grams of soil. For sandy coastal plain soils, the procedure will give a reasonably good estimate of the soil's CEC. If a soil contains free calcium carbonate, or has a cation exchange capacity greater than 10 meq/100 grams, this summation method may not correctly estimate the cation exchange capacity.
CALCULATIONS
To calculate the estimated cation exchange capacity, perform the following divisions and summation.
lbs. K/A ÷ 780 = meq K/100 gramslbs. Ca/A ÷ 400 = meq Ca/100 gramslbs. Mg/A ÷ 240 = meq Mg/100 gramslbs. Na/A ÷ 460 = meq Na/100 gramsExchangeable H‡ = meq H/100 grams
Total CEC = meq/100 grams
‡Exchangeable Hydrogen: meq/100 grams = (8.00 - buffer pH) X 8.
See the Adams-Evans buffer method for Lime Requirement - Exchangeable Hydrogen for details of the procedure.
For some soils, the failure to include Na will give slightly low results.
REFERENCES
- Hesse, P. R., 1972 A Textbook of Soil Analysis, pp. 91-105, Chemical Publishing Co., N. Y.
- Jackson, M. L. 1958, Soil Chemical Analysis, pp. 59-67, Prentice-Hall, Inc., Englewood, N. J.