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Colloidal Phosphorus in Surface Runoff and Water Extracts from Semiarid Soils of the Western United States

Benjamin L. Turner^*,, Mary A. Kay and Dale T. Westermann

USDA-ARS, Northwest Irrigation and Soils Research Laboratory, 3793 N. 3600 E., Kimberly, ID 83341

View larger version (13K): [in a new window]   Fig. 1. Concentrations of colloidal molybdate-reactive phosphorus (MRP) (µM) in surface runoff and soil-test P (mg P kg–1) for three calcareous arable soils from the semiarid western United States. Values are from sprinkler irrigation simulation experiments conducted on dry and wet soils.

View larger version (15K): [in a new window]   Fig. 2. Correlations between surface runoff concentrations of colloidal molybdate-reactive phosphorus (MRP) (µM) and colloidal Ca (µM) for the Portneuf soil (top), and colloidal Fe (µM) for the Warden soil (bottom). Runoff was generated by simulated sprinkler irrigation. Colloidal fractions were defined as particles between 1 µm and 1 nm in diameter.

View larger version (26K): [in a new window]   Fig. 3. Concentrations (µM) of molybdate-reactive phosphorus (MRP), Ca, Fe, and Mg in size fractions of water extracts of five representative semiarid arable soils of the western United States. Values for each filter pore size indicate the concentration passing through that filter.

View larger version (22K): [in a new window]   Fig. 4. Correlations between soil properties and concentrations of colloidal molybdate-reactive phosphorus (MRP) in water extracts of 12 semiarid arable soils of the western United States. Colloidal MRP concentrations are in µM (top row of figures) and as the proportion (%) of the filterable (<1 µm) MRP (bottom row of figures). Colloidal MRP was defined as P associated with particles between 1 µm and 0.3 nm in diameter.

View larger version (22K): [in a new window]   Fig. 5. Concentrations of colloidal molybdate-reactive phosphorus (MRP) (µM) in water extracts of three semiarid arable soils following acidification to different pH values with HCl. Colloidal MRP was defined as P associated with particles between 1 µm and 0.3 nm in diameter, and solutions were neutralized before filtration to prevent membrane damage.