CATION-EXCHANGE AND BUFFER POTENTIAL OF SASKATCHEWAN SOILS ESTIMATED FROM TEXTURE, ORGANIC-MATTER AND PH

Cation exchange capacity (CEC) data provide information on important c hemical attributes of soil (e.g., ability of soil to retain cations ag ainst leaching and to buffer pH). Measurements of CEC are expensive to perform. Further, since CEC is dependent on measurement pH, CEC data are difficult to interpret, especially in the case of soils whose fiel d pH is far removed from measurement pH. We analyzed a large data set (n = 1622), collected in support of soil survey activities in Saskatch ewan, to develop a method of estimating CEC as a function of pH and to establish relationships between soil buffer capacity and properties s uch as texture and organic matter content. A regression equation with organic C and clay as independent variables explained 86% of the varia bility in CEC measured using BaCl2 buffered al pH 8.2. The CECs (at pH 8.2) of organic matter and clay were estimated at 2130 and 510 mmol ( +) kg(-1), respectively. About 15% of exchange sites were not accounte d for by organic matter and clay and were assumed to reside in the fin e silt fraction. The CEC at field pH, ire., effective CEC (ECEC), was described (R-2 = 0.86**) by a function based on the assumption that t he ECECs of organic matter and clay increase linearly as pH increases to 8.2, where their values are 2130 and 510 mmol (+) kg(-1), respectiv ely. This relationship is especially useful because it enables soil CE C to be estimated at any pH based solely on organic matter and texture . Soil buffer capacity values were obtained by estimating the change i n soil ECEC (or titratable acidity) needed to produce a unit change in pH. Buffer strength of clay was low [similar to 30-50 mmol (+/-) kg(- 1) (pH unit)(-1)]. Our estimates of organic matter buffer capacity [si milar to 400 mmol (+/-) kg(-1) (pH unit)(-1)] were consistent with pub lished values. The results suggest that prairie soils that are low in organic matter may be susceptible to acidification even if clay conten t is relatively high.