Phosphorus losses in furrow irrigation runoff

Phosphorus (P) often limits the eutrophication of streams, rivers, and lake s receiving surface runoff. We evaluated the relationships among select ed soil P availability indices and runoff P fractions where manure, whey, or c ommercial fertilizer applications had previously established a range of soi l P availabilities on a Portneuf silt loam (coarse silty, mixed, superactiv e, mesic Durinodic Xeric Haplocalcid) surface-irrigated with Snake River wa ter. Water-soluble P, Olsen P (inorganic and organic P), and iron-oxide imp regnated paper-extractable P (FeO-P-s) were determined on a 0.03-m soil sam ple taken from the bottom of each furrow before each irrigation in fall 199 8 and spring 1999. Dissolved reactive phosphorus (DRP) in a 0.45-mum filter ed runoff sample, and iron-oxide impregnated paper-extractable P (FeO-P-w), total P, and sediment in an unfiltered runoff sample were determined at se lected intervals during a 4-h irrigation on 18.3-m field plots. The 1998 an d 1999 data sets were combined because there were no significant difference s. Flow-weighted average runoff DRP and FeO-P-w concentrations increased li nearly as all three soil P test concentrations increased. The average runof f total P concentration was not related to any soil P test but was linearly related to sediment concentration. Stepwise regression selected the indepe ndent variables of sediment, soil lime concentration, and soil organic P ex tracted by the Olsen method as related to average runoff total P concentrat ion. The average runoff total P concentration was 1.08 mg L-1 at a soil Ols en P concentration of 10 mg kg(-1). Soil erosion control will be necessary to reduce P losses in surface irrigation runoff.