STUDIES ON THE ADSORPTION OF SULFATE BY SOILS

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Abstract
Adsorption and desorption of soil sulfate were studied by measuring the changes in solution concentration when dilute solutions of calcium chloride containing potassium sulfate were shaken with soil. The change in adsorbed sulfate ([DELTA]QS) was plotted against solution concentration to give an adsorption isotherm. The relation was independent of the soil: solution ratio but was dependent on the time of shaking. Equilibrium was not reached within 48 hours. Increasing the Ca concentration above about 0.01 M increased the concentration of sulfate required for [DELTA]QS = 0 but had little effect on the activity. Decreasing the calcium concentration greatly increased the concentration and activity required for [DELTA]QS =0. The adsorption isotherm did not always fit a Langmuir form. The deviations suggested that increases in solution concentration activated further adsorption mechanisms. The ability to adsorb sulfate was correlated with the ability to adsorb phosphate. For both properties the partial regression coefficients for Fe extracted by citrate-dithionite were non-significant. Terms for extracted Al (citrate-dithionite) and for soil N were significant. The partial effect of N (i.e. when corrected for Al) was negative.