Sulfate desorption and mobility in soil can be of both environmental and agronomic importance. This study was conducted to describe the desorption of SO4 from agricultural soils under experimental conditions applicable to the movement of SO4 in overland flow and runoff waters. The kinetics of SO4 desorption from soil were investigated at five different water/soil ratios (10:1-400:1), five time periods (5-180 min), and four SO4 additions (0-200 mg kg-1). Sulfate desorption (Sd) was logarithmically related to contact time (t) and water/soil ratio (W) and was a linear function of the initial extractable SO4 content of soil (Se). The kinetics of SO4 desorption was described by the equation Sd = K Se t alpha W beta, where K, alpha, and beta are constants for a given soil. This equation has previously been shown to describe desorption of soil P and K. The kinetic constants, K (r2 = 0.88), alpha (r2 = 0.83), and beta (r2 = 0.88), were related to extractable soil Al content for 107 U.S. soils. The uniformity of equation constants for a given soil and the ease with which they can be estimated from extractable soil Al indicate the potential use of the equation in modeling SO4 desorption from soil and subsequent movement in surface runoff water.
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