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Plant Availability of Phosphorus in the Water-Insoluble Fraction of Commercial Triple Superphosphate Fertilizers

Dep. of Agronomy and Soils, Auburn Univ., Auburn, AL 36849-5412

F. J. Sikora

Tennessee Valley Authority, F2421 National Fertilizer Development Center, Muscle Shoals, AL 35660

*Corresponding author.

ABSTRACT

Water-insoluble complexed P fractions can form during the production of phosphate fertilizers, and there is some concern regarding the potential impact of these chemical impurities on fertilizer performance. A greenhouse study was conducted to evaluate the plant availability of P in water-insoluble residues from five commercial triple superphosphate (TSP) fertilizers. Fertilizers manufactured from rock-phosphate sources in Florida, North Carolina, Idaho, and Morocco were evaluated. Each fertilizer was washed with deionized water to remove its water-soluble P. The resulting water-insoluble fertilizer residues, reagent-grade monocalcium phosphate [MCP: Ca(H₂PO₄)₂] and reagent-grade dicalcium phosphate (DCP: Ca-HPO₄) were applied to a Mountview silt loam soil (fine-silty, siliceous, thermic Typic Paleudult) to supply 0, 10, 20, 40, 60, and 80 mg P kg^–1 soil. Sorghum-sudangrass [Sorghum bicolor (L.) Moench] was harvested for forage yields at 21 and 42 d after planting. Each P source gave different (P 0.01) yield and P-uptake responses to the rate of the sum of the water-soluble and the citrate-soluble P applied. Water-insoluble fertilizer residues were a better source of P than reagent-grade DCP, but they were not as effective as reagent-grade MCP. Results from this study demonstrate that the plant availability of P in the water-insoluble fraction of TSP fertilizers is source dependent, and that P in this fraction is not as available as P in pure MCP. The sum of the water-soluble and the citrate-soluble P overestimated available P in the fertilizer residues.

Contribution of Dep. of Agronomy and Soils and Alabama Agric. Exp. Stn. Supported in part by the Tennessee Valley Authority (Contract no. TV-69273A) and the Potash and Phosphate Institute.

Received for publication November 27, 1989.

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