Sharpley: Kinetics of Sulfate Desorption

Sulfate desorption and mobility in soil can be of both environmental and agronomic importance. This study was conducted to describe the desorption of SO 4 from agricultural soils under experimental conditions applicable to the movement of SO 4 in overland flow and runoff waters. The kinetics of SO 4 desorption from soil were investigated at five different water/soil ratios (10:1-400:1), five time periods (5-180 mm), and four SO 4 additions (0-200 mg kg ). Sulfate desorption ( Sd ) was logarithmically related to contact time (1) 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 S, = K S, t W, where K, a, and 0 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), a (r2 0.83), and (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 SO 4 desorption from soil and subsequent movement in surface runoff water. T HE EFFECT OF ACID PRECIPITATION and SO4 deposition on terrestrial and aquatic ecosystems has received considerable national and international attention (Council for Agricultural Science and Technology, 1984; Linthurst, 1984; McFee, 1980; National Academy of Sciences, 1981). Of particular importance is the increase in SO 4 mobility in some soils, which can lower the pH of percolating and runoff waters (Johnson and Cole, 1977; Ulrich et al., 1980) and subsequently result in a reduction in stream and lake water pH (Burns et al., 1981; Drablos and Tollan, 1980; Wright and Gjessling, 1976). Further, leaching losses of SO4 coupled with low inputs have resulted in S deficiencies in soils of the humid tropics (BolleJones, 1964; Hasan et al., 1970; McClung et al., 1959) as well as in and (Conrad, 1950; Neller, 1925) and temperate regions (Bertramson et al., 1950; Ensminger, 1954). With regard to these environmental and agronomic aspects, the adsorption of SO4 by soil has been investigated. Although SO4 adsorption has been shown to be influenced by soil pH (Hue et al., 1984; Singh, 1984b; Bolan et al., 1986), Al and Fe sequioxides (Aylmore et al., 1967; Chao et al., 1964; Singh, 1984a), and USDA-AR5 Water Quality and Watershed Research Lab., P.O. Box 1430, Durant, OK 74702-1430. Received 16 Jan. 1990. *Corresponding author. Published in Soil Sci. Soc. Am. J. 54:1571-1575 (1990). fertilizer P application (Haron and Hanson, 1988; Kamprath et al., 1956), the mechanism of adsorption is debated. Ligand-exchange mechanisms, which result in covalent surface bonds (Hingston et al., 1967; Parfitt, 1978; Rajan, 1979) and electrostatic forces (Yates and Healy, 1975; Marsh et al., 1987) have been suggested. Limited information is available, however, on the kinetics of SO 4 reactions in soils (Hodges and Johnson, 1987; Singh 1984c; Sparks, 1986). The adsorption of 50 4 can be rapid, with Rajan (1978) reported adsorption by hydrous alumina to be 95% complete within 60 mm. For soils containing both organic and inorganic constituents, however, Singh (1984c) found that 85 and 62% of added SO 4 was adsorbed by an Fe Podzol and Brown Earth, respectively, in 24 h. The desorption of SO4 from soil (Chao et al., 1962) and kaolinite (Aylmore et al., 1967) is also rapid. More recently, Hodges and Johnson (1987) described the kinetics of both SO 4 adsorption and desorption for Cecil soil (clayey, kaolinitic, thermic Typic Kanhapludult) by Elovich, parabolic diffusion, and shell progressive particle and film diffusion equations. Little information is available, however, on the kinetics of SO4 desorption from a wide range of soils under experimental conditions applicable to the movement of SO4 in overland flow and runoff waters. For this purpose, the kinetics of SO4 desorption with water, especially during short periods of soil—water interaction at a wider range of water/soil ratios than those of earlier studies, is needed. Consequently, the power-form equation describing the desorption of P and K from soil (Sharpley et al., 1981; Sharpley, 1987), which assumes nonlinear ion diffusion, is proposed to describe SO4 desorption from soil over short time periods (<180 mm) and wide water/soil ratios (10:1 to 400:1). The equation has the form