Cation exchange capacity measurements

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Abstract
Soil cation exchange capacity (CEC) measurements are important criteria for soil fertility management, waste disposal on soils and soil taxonomy. An attempt was made to compare CEC values for arable Ultisols from the humid region of the USA as determined by procedures varying widely in their chemical conditions during measurement. Exchangeable cation quantities determined in the course of 2 of the CEC procedures were evaluated. The 6 procedures evaluated were: summation of N NH4OAc [ammonium acetate] (pH 7.0) exchangeable Ca, Mg, K, and Na plus BaCl2 - TEA [triethanolamine] (pH 8.0) exchangeable acidity; N Ca(OAc)2 (pH 7.0) saturation with Mg(OAc)2 (pH 7.0) displacement of Ca2+; N NH4OAc (pH 7.0) saturation with NaCl displacement of NH4+; N MgCl2 saturation with N KCl displacement of Mg2+; compulsive exchange of Mg2+ for Ba2+; and summation of N NH4OAc (pH 7.0) exchangeable Ca, Mg, K, and Na plus N KCl exchangeable Al. The unbuffered procedures reflect the pH dependent CEC component to a greater degree than the buffered methods. The compulsive exchange and the summation of N NH4OAc exchangeable cations plus N KCl exchangeable Al procedures gave CEC estimates of the same magnitude that reflect differences in soil pH and texture. The buffered procedures, particularly the summation of N NH4OAc exchangeable cations plus BaCl2 - TEA (pH 8.0) exchangeable acidity, indicated inflated CEC values for these acid Ultisols that are seldom limed above pH 6.5. Exchangeable soil Ca and Mg levels determined from extraction with 0.1 M BaCl2 wre consistently greater than values for the N NH4OAc (pH 7.0) extractions. The Ba2+ ion is apparently a more efficient displacing agent than the NH4+ ion. Also, the potential for dissolving unreacted limestone is greater for the Ba2+ procedures than in the NH4+ extraction.