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 Pormeuf 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 lowerfield 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. INTRODUCTION Groundwater seepage supplies a portion of flow to irrigation return and surface waters. Knowledge of these additions is needed to manage phosphorus (P) total mean daily loads in streams draining irrigated crop lands. In the past, water percolating through surface-irrigated calcareous silt loam soils was thought to make insignificant P contributions to groundwater. However, recent studies suggest that P may be more mobile in calcareous soils than originally expected. It is not known whether an alternative furrow irrigation practice, which uses polyacrylamide (PAM) for erosion control (Lentz et al., 1992; Lentz and Sojka, 1994) may influence P losses. OBJECTIVES We designed vacuum-assisted soil water percolation samplers that were used to monitor percolation water flux and P concentrations in water draining below the crop root zone in a furrow-irrigated Portneuf silt loam. Two furrow irrigation approaches were compared, conventional and polyacrylamide-managed. MATERIALS AND METHODS Plots were located near Kimberly, ID on Portneuf Silt Loam, 1.5% slopes (coarse-silty, mixed, superactive, mesic, Durinodic Xeric Haplocalcids). All plots had received dairy manure regularly during the period from 1969 to 1989, ie. 10 to 20 ton acre -I (dry wt.) were applied in two out of every seven years. The irrigation water came from the Snake R. The complete randomized block design included three irrigation treatments, but we monitored soil water in only two of the three treatments. Control treatment furrows were irrigated with 4 gpm (15 L