Phosphorus Losses in Furrow Irrigation Runoff

ABSTRACT Phosphorus (P) often limits the eutrophication of streams, rivers, and lakes receiving surface runoff. We evaluated the relationships among selected soil P availability indices and runoff P fractions where manure, whey, or commercial fertilizer applications had previously established a range of soil P availabilities on a Portneuf silt loam (coarse‐silty, mixed, superactive, mesic Durinodic Xeric Haplocalcid) surface‐irrigated with Snake River water. Water‐soluble P, Olsen P (inorganic and organic P), and iron‐oxide impregnated paper–extractable P (FeO‐Ps) were determined on a 0.03‐m soil sample taken from the bottom of each furrow before each irrigation in fall 1998 and spring 1999. Dissolved reactive phosphorus (DRP) in a 0.45‐μm filtered runoff sample, and iron‐oxide impregnated paper–extractable P (FeO‐Pw), total P, and sediment in an unfiltered runoff sample were determined at selected intervals during a 4‐h irrigation on 18.3‐m field plots. The 1998 and 1999 data sets were combined because there were no significant differences. Flow‐weighted average runoff DRP and FeO‐Pw concentrations increased linearly as all three soil P test concentrations increased. The average runoff total P concentration was not related to any soil P test but was linearly related to sediment concentration. Stepwise regression selected the independent variables of sediment, soil lime concentration, and soil organic P extracted by the Olsen method as related to average runoff total P concentration. The average runoff total P concentration was 1.08 mg L−1 at a soil Olsen P concentration of 10 mg kg−1 Soil erosion control will be necessary to reduce P losses in surface irrigation runoff.