Runoff Phosphorus Losses as Related to Soil Test Phosphorus and Degree of Phosphorus Saturation on Piedmont Soils Under Conventional and No-Tillage

Abstract

Abstract Elevated soil phosphorus (P) concentrations on agricultural land from the application of P in fertilizers and animal manures have increased the potential for excessive P losses in runoff to nutrient-sensitive waters. This study was conducted to determine P losses in runoff from agricultural land in the Piedmont region of the southeastern U.S. with varying soil P levels resulting from past applications of broiler litter and inorganic P fertilizers. The correlations between soil P content and P in runoff can be used to validate and develop P loss-assessment tools for Piedmont soils of the southeastern US. Rainfall simulation at rates of 6.35 cm hr−1 were utilized to collect runoff samples from cropland under conventional tillage (CT) and no tillage (NT) with a range of initial P concentrations. Runoff samples were collected at 5-min intervals for 30 min and analyzed for reactive P (RP), algal-available P (AAP), and total P (TP). Concentration of RP in runoff from CT and NT plots was positively correlated with Mehlich-3 extractable P (M3 STP) (r 2 = 0.70 and 0.67, respectively) and oxalate extractable degree of P saturation (DPS) (r 2 = 0.62 and 0.57, respectively). A Mehlich-3 extractable P concentration of 294 mg P kg−1 and a DPS of 65% corresponded to 1 mg RP L−1 in runoff from NT plots. There was a significant linear relationship between M3 STP concentration and DPS on CT and NT plots. Results of this study indicate that both M3 STP concentration and DPS can be used to help predict P losses from a typical Piedmont soil managed with CT and NT cultivation. Overall, these soil tests predicted P concentration in runoff more effectively than P mass losses in runoff.