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Dissolution of Phosphate Rocks in a Flooded Acid Soil¹

ABSTRACT

Seven phosphate rocks from widely separated locations were used to investigate the dissolution of phosphate rocks in an acid, P-deficient soil (Mountview silt loam, pH 4.8) under waterlogged conditions. The phosphate rock was added at a rate of 400 ppm P in the soil. The objective of the study was to investigate the way in which the P concentration in the soil solution equilibrated with the various phosphate rocks was related to the apatite composition and the citrate solubility of the phosphate rocks used.

The concentration of P in the soil solution equilibrated with various phosphate rocks during incubation varied considerably among sources of the rock, ranging from 0.03 to 0.35 ppm in 1 week and 0.04 to 0.11 ppm in 10 weeks after the soil was flooded. The solubility differences of various phosphate rocks decreased as incubation time increased.

The P concentrations in the soil equilibrated with phosphate rocks reached a maximum 1-3 weeks after the soil was flooded. There was a linear relationship (r = 0.93) between the logarithm of the maximum P concentrations in the soil solution equilibrated with phosphate rocks and the degree of carbonate substitution for phosphate in the apatite structure of the phosphate rocks used. A highly significant linear correlation (r = 0.95) was found between the maximum P concentration in the soil solution and the citrate solubility of the phosphate rocks used. The correlation, however, decreased to 0.82 in 10 weeks after the flooding. Apparently, factors other than apatite composition were also affecting the dissolution of phosphate rocks in the soil solution after prolonged incubation.

¹ Contribution from the Fertilizer Technology Div., Int. Fest. Develop. Center, Muscle Shoals, AL 35660

² Research Soil Chemist.

Received for publication May 5, 1977. Accepted for publication August 9, 1977.