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

Abstract

AbstractWater‐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.