We examined interactions of lime and phosphate and subsequent adsorption of phosphate, sulfate, and molybdate by a Spodosol, using incubation and isotherm studies.
Incubation of the soil with lime decreased the quantity of added phosphate that could be extracted with sodium bicarbonate or anion exchange resin. When lime and phosphate were added simultaneously, liming increased phosphate adsorption. When the soil was incubated with lime and the moist soil then equilibrated (1:10 soil-solution ratio) with 5-üg/ml solutions of Mo, P, or S, liming increased subsequent adsorption of phosphate, had no effect on that of sulfate, but decreased that of molybdate. It is suggested that liming resulted in the formation of new, highly active, adsorbing surfaces in the soil through the precipitation of exchangeable aluminum as amorphous polymeric hydroxy-Al cations. The formation of these new surfaces had the greatest effect on phosphate adsorption and the least on that of molybdate, because in general, the magnitude of the decrease in specific adsorption of anions by soil surfaces as pH increases follows the order phosphate > sulfate > molybdate.
Incubation of the soil with lime, phosphate, or both followed by air-drying, increased the net negative soil charge and clearly decreased subsequent adsorption of phosphate, sulfate, and molybdate by the soil. Evidently, air-drying significantly altered the surface charge characteristics of the limed soil, for liming increased phosphate adsorption by moist soils, but decreased that by air-dried soil. Crystallization of the amorphous hydroxy-Al surfaces, during air-drying, may explain the different adsorption characteristics of moist and air-dried limed soil with respect to phosphate and to a lesser extent sulfate.
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