Percolation phosphorus losses in calcareous furrow-irrigated soils

Groundwater seepage contributes to irrigation return flows and surface waters. Knowledge of phosphorus (P) concentrations and loads in percolation water that enters the vadose zone and groundwater is necessary to assess and manage water quality. We measured the P in water leached below the crop root zone in a furrow-irrigated Portneuf silt loam. Vacuum operated percolation samplers were used to monitor soil water flux, and molybdate-reactive-P (MRP) and total P concentrations in leachate during the cropping season. The samplers were placed at 4 ft depths at 100 ft and 500 ft across a 590-fi-long corn field that was furrow irrigated using either conventional or PAM treatment. We found no irrigation treatment effects on cumulative amounts of water or P leached The MRP concentrations in leachate ranged from 0.01 to 2.0 ppm and averaged 0.58 ppm during three irrigation seasons. The MRP concentrations in lower-field leachate were nearly 10X greater than those from the upper-field. The P mass losses during the irrigation season were less than 0.1 lb acre -J . The P concentrations in leachate were 2 to 20 times greater than ortho-P equilibrium concentrations measured in the subsoil water (i.e. dilute CaC12 extractable ortho-P). P concentrations in drainage water appeared to be reflective of percolate volume and soil chemistry.