SULFATE SORPTION BY ACID FOREST SOILS

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Abstract
To examine the fate of adsorbed sulfate by acid forest soils and its potential losses due to acid precipitation, desorption of adsorbed sulfate was determined as a function of time, soil:solution ratio, desorbing ion, pH, and amount of adsorption. Desorption of sulfate was rapid and completed within half an hour. Sulfate desorption by water, dilute HCl and Ca(NO3)2 was inversely related to the ability of the soils to adsorb SO4-2, but it increased with pH and the amount of adsorption. The very low desorption of SO4-2 in the Fe-podzol by either water or Ca(NO3)2 at pH below 5.0 suggests that the adsorbed SO4-2 will be highly resistant to acid leaching, but it may also be less available to plants. The proportion of specifically adsorbed SO4-2 was higher in the Fe-podzols than in the semipodzol and brown-earth, but it decreased with increasing pH and the amount of adsorption. Sulfate desorbed by Ca(H2PO4)2 was least affected by the nature of adsorbing complex, pH, and amount of adsorption, indicating that phosphate is able to desorb SO4-2 adsorbed due to either permanent or pH-dependent charges. The possible mechanism of SO4-2 by these soils is discussed.