Phosphorus runoff during four years following composted manure application.

Repeated manure application can lead to excessive soil test P (STP) levels and increased P concentration in runoff, but also to improved water infiltration and reduced runoff. Research was conducted to evaluate soil P tests in prediction of P concentration in runoff and to determine the residual effects of composted manure on runoff P loss and leaching of P. The research was conducted from 2001 to 2004 under natural runoff events with plots of 11-m length. Low-P and high-P compost had been applied during the previous 3 yr, resulting in total applications of 750 and 1150 kg P ha(-1). Bray-P1 in the surface 5 cm of soil was increased from 16 to 780 mg kg(-1) with application of high-P compost. Runoff and sediment losses were 69 and 120% greater with no compost than with residual compost treatments. Runoff P concentration increased as STP increased, but much P loss occurred with the no-compost treatment as well. Agronomic soil tests were predictive of mean runoff P concentration, but increases in STP resulted in relatively small increases in runoff P concentration. Downward movement of P was not detected below 0.3 m. In conclusion, agronomic soil tests are useful in predicting long-term runoff P concentration, and risk of P loss may be of concern even at moderate soil P levels. The residual effect of compost application in reducing sediment and runoff loss was evident more than 3 yr after application and should be considered in P indices.