Source‐Related Transport of Phosphorus in Surface Runoff

Continual application of mineral fertilizer and manures to meet crop production goals has resulted in the buildup of soil P concentrations in many areas. A rainfall simulation study was conducted to evaluate the effect of the application of P sources differing in water‐soluble P (WSP) concentration on P transport in runoff from two grassed and one no‐till soil (2 m 2 plots). Triple superphosphate (TSP)–79% WSP, low‐grade single superphosphate (LGSSP)–50% WSP, North Carolina rock phosphate (NCRP)–0.5% WSP, and swine manure (SM)–30% WSP, were broadcast (100 kg total P ha −1 ) and simulated rainfall (50 mm h −1 for 30 min of runoff) applied 1, 7, 21, and 42 d after P source application. In the first rainfall event one d after fertilizer application, dissolved reactive P (DRP) and total P (TP) concentrations of runoff increased ( P < 0.05) for all soils with an increase of source WSP; with DRP averaging 0.27, 0.50, 14.66, 41.69, and 90.47 mg L −1 ; and total P averaging 0.34, 0.61, 19.05, 43.10, and 98.06 mg L −1 for the control, NCRP, SM, LGSSP, and TSP, respectively. The loss of P in runoff decreased with time for TSP and SM, such that after 42 d, losses from TSP, SM, and LGSSP did not differ. These results support that P water solubility in P sources may be considered as an indicator of P loss potential.