Effect of Water-Soluble Phosphorus on the Effectiveness of Triple Superphosphate Fertilizers
1995
Mullins, G. L. | Sikora, F. J. | Williams, J. C.
Impure phosphate rock results in the formation of P-containing impurities having low water solubility in commercial triple superphosphate (TSP) fertilizers, which may influence their effectiveness. A greenhouse study was conducted to determine the level of water-soluble P needed in TSP to maximize plant P uptake and yield when applied to a slightly acid soil (pH = 6.4). Fertilizer impurities were isolated from four commercial TSP fertilizers as a water-washed fraction by washing the whole fertilizers with deionized water. Fertilizers with varying water-soluble P were simulated by mixing reagent-grade monocalcium phosphate (MCP; ≈ 100% water soluble) with water-washed fertilizer fractions or reagent-grade dicalcium phosphate (DCP; ≈ 1.2% water-soluble P). The mixtures contained 0, 20, 40, 60, and 80% of the available P (available = water-soluble + ammonium citrate soluble P) as MCP. An additional treatment received 100% water-soluble P as MCP. Mixtures were applied to supply 50 mg available P kg⁻¹ soil. Sorghum-sudangrass [Sorghum bicolor (L.) Moench] was harvested 21 and 42 d after planting. Forage yield and P uptake were affected by P source and water-soluble P content. Compared with using a 100% water-soluble P source (MCP), individual TSP fertilizer materials required 52 to 66% water-soluble P to attain 90% of the dry matter yield and 41 to 51% water-soluble P to attain 90% of P uptake. Results of this study demonstrate that current levels of “water-insoluble P” in commercial TSP sources are not affecting their performance when added to slightly acid soils.
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