CLIMATIC CONTROL OF pH BUFFERING IN THE A AND B HORIZONS OF UNITED STATES SOILS

In previous studies, models of the climatic controls on pH in the A (pHA) and B horizons (pHB) of U.S. soils were developed which differ in the functional forms and variables incorporated into each model. This study suggests that the differences in the models are explained by climate's control over the soil properties that determine pH buffering. Climatic control of pHA and pHB buffering in most acid soils is through its control over clay mineralogy. However in very acid soils, organic matter content (OM) is a stronger buffer of pHA than is clay mineralogy because in these highly leached soils, OM contents are very high and most OM is concentrated in the A horizon. In basic soils, climate dominates buffering through its control over the chemical reactions involving calcium carbonate (CaCO₃) formation. The pHA model suggests that in basic soils as moisture decreases CaCO₃ content increases causing pHA buffering to increase. In contrast, the pHB model suggests that as moisture decreases pHB buffering decreases. Although leaching of basic soils is limited, when it occurs a highly-concentrated pulse of calcium (Ca²⁺) is illuviated to the B horizon, causing pHB to increase. As leaching decreases, the magnitude of this pulse of Ca²⁺ increases because more Ca ²⁺ is available for leaching and depth of leaching of Ca²⁺ decreases. Studies of basic soils support only the results of the pHA model, but they are based mainly on laboratory studies which do not consider the influence of climate on B horizon chemistry. [Key Words: Soil acidity, pH buffering, climate, modeling, U.S. soils.]