PERTANIKA JOURNAL OF TROPICAL AGRICULTURAL SCIENCE

The cation exchange capacity (CEC) of soil is an important criteria for assessing soil fertility beside its use in soil classification. Seven methods, namely, (1) BaCl2 - triethanolamine of BaCl2 - TEA (pH 8.2), (2) NH4OAc (pH 7.0) - leaching, (3) NH4OAc (pH 7.0) - shaking, (4) compulsive exchange method of Gillman (1979) (CECC£), (5) modified compulsive exchange method of Gillman (1986) (GECJ, (6) Summation of Ca from method 5 with 1 MNHJSI03 exchangeable Al (CEC p and (7) summation ofNH4OAc (pH 7.0) exchangeable bases with 1 M KCl exchangeable Al (CEC J, were used to determine and compare the CEC values of five acid tropical soils. All methods gave different CEC values which followed the order BaCl2 - TEA > NH4OAc shaking = NH4OAc leaching > CEC = CEC > CEC— = CECR. Methods with pH conditions close to field situations gave much lower CEC values than the buffered methods. The buffered methods generate charge on the variable-charge colloids, thus resulting in inflated CEC values, while the unbuffered methods do not. There is a high correlation between BaCl2 - TEA and NH4OAc (pH 7.0) leaching method; CEC(lt and CECsum; and, CECR and CEC(otal. Amongst the methods evaluated, the NH4OAc (pH 7.0) leaching is recommended in routine soil analyses for classification purposes while CEC is recommended for agronomic evaluation.