Comparison of Highly-Weathered Acid Soil CEC Determined by NH4OAc (pH = 7.0) Exchange Method and BaCl2-MgSO4 Forced-Exchange Method

Cation exchange capacity (CEC) is one of the most important properties of soils. The NH4OAc (pH = 7.0) exchange method is usually recommended to determine CEC (CEC1) of all soils with different pH values, particularly for studies on soil taxonomy. But comparatively the BaCl2-MgSO4 forced-exchange method is more authentic in determining CEC (CEC2) of tropical and subtropical highly-weathered acid soils. But so far little is known about the difference between CEC1 and CEC2. In this study, the physiochemical data of 114 acid B horizon soils from 112 soil series of tropical and subtropical China were used, CEC1 and CEC2 were determined and compared, the influencing factors were analyzed for the difference between CEC1 and CEC2, and then a regression model was established between CEC1 and CEC2. The results showed that CEC2 was significantly lower than CEC1 (p 2 was 14.76% - 63.31% with a mean of 36.32% of CEC1. In view of the contribution to CEC from other properties, CEC2 was mainly determined by pH (45.92%), followed by silt (21.05%), free Fe2O3 (17.35%) and clay contents (12.76%), CEC1 was mainly decided by free Fe2O3 content (40.38%), followed by pH (28.39%) and silt content (27.29%; and the difference between CEC1 and CEC2 was mainly affected by free Fe2O3 (50.92%), followed by silt content (26.46%) and pH (21.80%). The acceptable optimal regression model between CEC2 and CEC1 was established as CEC2 = 2.3114 × CEC11.1496 (R2 = 0.410, P 2-MgSO4 forced-exchange method is recommended in determining CEC of the highly-weathered acid soils in the tropical and subtropical regions.