Effect of organic matter on CEC due to permanent and variable charges in selected temperate region soils

Cation exchange capacities due to permanent charges (CECp) and variable charges (CECv) and at pH 8.2 (CECt) were determined in soil samples before and after removal of organic matter with H2O2 to study the effect of interactions between organic matter and clay on the different types of CEC. Five soils with differing organic matter contents and different clay mineralogies were selected and used, e.g. Dubuque, Tubac, Tama, Greenville and Houston Black series. The results indicated that the lowest CECp (2.9 mequiv./100 g soil) was detected in the Ap horizon of Greenville soil with a low organic matter content and kaolinitic mineralogy. The highest CECp (11.3 mequiv./100 g soil) in the A horizon was measured in the Houston Black soil also with low organic matter content but which contained montmorillonitic clay. Dubuque, Tubac and Tama A horizons, with mixed mineralogy, had CECp values (4.6–6.5 mequiv./100 g soil) between those exhibited by the Greenville and Houston Black soils. After removal of organic matter, the CECp of the Houston Black soil increased in value, indicating that the permanent charge of the montmorillonitic clay has been blocked by the interaction of the organic matter with the clay. This interaction between two soil components causing a decrease in CECp is known in plant science as antagonism. On the other hand, removal of organic matter from soils with mixed mineralogy resulted in a decrease in CECp. The interactions between organic matter and clay in these soils yielded clay—organic complexes with a synergistic effect on permanent charges, which produced the higher CECp values. In the Greenville Ap horizon, destruction of organic matter had no effect on CECp. Based on absolute values, the A horizons of the soils had low CECv's, with the Houston Black exhibiting relatively the highest value (24.0 mequiv./100 g soil). The Ap horizon of the Greenville soil had a CECv (6.7 mequiv./100 g soil), which was 3.5 times smaller than that of the A1p horizon of the Houston Black soil. When the percentage of the CECv contributing to the CECt was studied, however, it appeared that in the Greenville Ap horizon 70% of the CECt was attributed to the CECv, compared to 68% in the Houston Black A1p horizon. The conclusion was made that variable charges were equally prominent in the soils studied. In the absence of organic matter, %CECv in the A