Effect of Water‐Soluble Phosphorus on the Effectiveness of Triple Superphosphate Fertilizers

Abstract

AbstractImpure 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.