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Reactions of Phosphate Rocks, Rhenania Phosphate, and Superphosphate with an Acid Soil¹

ABSTRACT

The amounts of water-soluble P in various phosphate materials after their reaction with Weston soil (pH 4.5) at a rate of 400 ppm P followed the order: Rhenania phosphate > CSP > North Carolina rock > Araxa rock. Addition of 150 ppm Si in the form of Na₂SiO₃ to the soil treated with CSP significantly decreased the P-retention capacity of the soil. The water-soluble P level was maintained at 100–150 ppm in the soil treated with CSP and silicate during the whole course of incubation (10 weeks). Addition of silicate, however, did not eliminate P retention completely.

Liming the soil with Ca(OH)₂ decreased the water-soluble P in the soil treated with CSP. In contrast, soluble silicate increased the water-soluble P as the rate of application increased, despite the fact that soil pH increased. The amounts of water-soluble P in the soil treated with Rhenania phosphate and CSP together at the ratio of 1:1 (as P) were found to be greater than the calculated sum of water-soluble P in the soil treated with Rhenania phosphate and CSP separately. This suggests that the silicate in Rhenania phosphate decreased the soil P-retention capacity and thus increased the level of water-soluble P in the soil. It appears that Rhenania phosphate should be a potential P fertilizer source for those tropical acid soils with relatively high P-retention capacity in some developing countries, provided its cost can be justified.

¹ Contribution from the Fertilizer Technology Division, International Fertilizer Development Center, Muscle Shoals, Al 35660.

² Research Chemist (Soils).