Phosphorus Source and Soil Properties Effects on Phosphorus Availability

Phosphorus (P) application in excess of crop needs has increased the concentration of P in soils and has been linked to freshwater eutrophication leading to P-based nutrient management. However, insufficient data are available relating soils properties, P-source solubility, and rate and time of application to develop sound guidelines. We investigated P release from nine soils ranging in chemical and physical properties when P sources differing in water-soluble P (WSP) are applied. Triple superphosphate (79% WSP), low-grade single superphosphate (50% WSP), North Carolina rock phosphate (NCRP; 0.5% WSP), and swine manure (60% WSP) were applied at 50 and 100 kg total P ha−1 to nine soils ranging in chemical and physical properties and incubated for 14 and 84 days. Phosphorus sorption maximum ranged among soils (8-270 mg kg−1), reflecting differences in the capacity of soils to bind added P. The concentration of Mehlich-3 P was greater (P < 0.05) for the most soluble P source (triple superphosphate) 14 days after soil incubation and increased as application rate increased from 50 to 100 kg P ha−1. In general, the Mehlich-3 P concentration of soil incubated with NCRP increased with increasing time of soil incubation, whereas for the other sources decreased Mehlich-3 P with incubation time, reflecting the slow release of P from NCRP. These results demonstrate that the type of P added, as well as soil properties and time and rate of application, can influence soil P availability. This information is important in evaluating short- and long-term effects of P-source application on soil P fertility and release to soil and runoff water.