Effects of Phosphorus Source and Water Quality on the Movement of Phosphorus through Sandy Soil.

Limited work has been performed to assess the potential for phosphorus (P) to leach in the Central Sands of Wisconsin. These irrigated sandy soils have elevated soil test P levels primarily from application of fertilizer to potatoes and vegetables.  Initial investigation by deep coring revealed that at a depth of 0.6-0.9 m Bray P was 25-30 mg kg^-1 greater in field areas compared to non field areas.  A laboratory column leaching study was conducted with the objectives of evaluating: 1) P source and water quality effects on dissolved reactive P (DRP) and total P (TP) in leachate water and rhizon-soil moisture sampler DRP (RMS DRP), 2) treatment effects on soil test P at various depths, and 3) the number of leaching events required to leach P. Bray 1 P ranged from 60-250 mg kg^-1 in these four coarse textured soils (one with a history of manure application). Columns were 0.15 m long and 0.10 m diameter, soil was packed to a density of 1300 kg m^-3.  Rhizon moisture samplers were inserted into the column 7.6 cm below the soil surface.  Three P sources (0.64 mg P cm^-2 applied as monoammonium phosphate (MAP) or dairy manure, or no P) were applied to the surface of the columns. Three water sources (two irrigation waters with different iron and calcium concentrations and synthetic rainwater) were applied during four leaching events. Elevated levels of RMS DRP were found for MAP but not for the other treatments. Leachate DRP was not always greater for MAP.  In 3 of 4 soils, leachate DRP was above 0.10 mg L^-1 where no P was applied.