Approach
Surface soil samples were obtained from 18 irrigated fields in the western U.S. They were physically and chemically characterized with standard procedures after air drying. Soil P fractions were characterized by NaHCO3 extractable inorganic and organic P after persulfate digestion, water extractable P, iron-oxide impregnated paper extractable P as an estimate of bioavailable P, total P by NaOH fusion, inorganic and organic P by the Saunders & Williams method. Soil pH was determined in a saturated paste (Table 1). Weighed soil, 1, 2, 4 or 10 g, was placed in 2 L of reverse osmosis water (RO), a 50:50 mix of RO and well water to simulate Snake River irrigation water (IRR), or 0.01 M CaCl2 contained in round plexiglass containers (24 cm by 19.5 cm diameter). These were then rotated at 12 rpm in a constant temperature room at 20 degrees C (Figure 1). During continuous rotation, 0.06 L of the soil-solution was withdrawn at 5, 15, 45, 120 and 240 elapsed minutes and passed through a 1.0µm filter, with one-half refiltered through a 0.2µm. Dissolved reactive P (DRP) was determined on the filtrates by the molybdenum-blue method. Data were analyzed by standard regression techniques. Our discussion will present results at 5 and 240 minutes on the 0.2µm filtered samples in an effort to minimize P associated with particulate or colloidal materials.