A comparison of calcium-magnesium phosphate and superphosphate as phosphate fertilizers on acid soils

Abstract

Fused calcium magnesium phosphate and superphosphate were compared as fertilizers for subterranean clover growing on four phosphorus deficient soils in a pot culture experiment. The thermally produced phosphate fertilizer was least effective on the podzolic soil developed on granite, but its effectiveness, relative to superphosphate, increased in the order podzolic soil developed on sediment, chocolate soil developed on basalt, until, on the krasnozem developed on basalt, calcium magnesium phosphate was superior to superphosphate as a phosphate fertilizer. This increase in relative effectiveness of the thermal phosphate was related to the increase in phosphorus sorption on these soils. Dithionite extractable iron was the significant common factor influencing their sorption capacity, and substituting these values for sorption capacity over this range of soils improved the correlation with the relative effectiveness of the thermal phosphate. There is a place for this type of thermal phosphate in pasture production on high phosphate sorbing soils, where its superior effectiveness is probably due to the slower release of phosphorus from this form, rather than to any specific reaction of the fertilizer with the soil.