Phosphorus loss in runoff from long-term continuous wheat fertility trials

Some wheat (Triticum aestivum L.) farmers in the southern Great Plains routinely apply P fertilizer without soil testing. This practice may lead to P buildup in soils, hence, increased P runoff potential, making soil P management of concern in continuous wheat production. At present, there is also debate over the nature of P loss trends, particularly whether soil P release to runoff can be described generally, across a broad range of soils, or is soil-specific. Paired 1 m by 2 m runoff plots were established on three existing long-term continuous winter wheat fertility experiments. Two experiments have received annual fertilizer P application at different rates since 1970 (0-44 kg P ha - 1 ), and the third received a one-time P application at much higher rates in 1977 (0-587 kg P ha - 1 ). Rainfall-runoff experiments were conducted following the National P Research Project protocol. Simulated rain (75 mm h - 1 ) produced 30 min of runoff from plots with different soil test P levels. Soil Mehlich-3 P (M3P) ranged from 11.5 to 130 mg kg - 1 and water-soluble P (WSP) ranged from 0.70 to 15.7 mg kg - 1 . Runoff total P and dissolved reactive P (DRP) concentrations ranged from 0.47 to 1.5 and 0.03 to 0.70 mg L - 1 , respectively. Dissolved reactive P in runoff was significantly related to M3P, WSP, and ammonium oxalate P saturation index (PSI o x ) for combined soils as well as for individual soil series. Significant differences (p < 0.05) among the slopes of the regressions for the DRP-M3P, DRP-WSP, and DRP-PSL, relationships indicate that the relationships are soil specific. This study highlights the need for soil specific management to protect water quality.