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This article in JEQ

  1. Vol. 33 No. 4, p. 1464-1472
    Received: Sept 17, 2003

    * Corresponding author(s): bturner@ifas.ufl.edu
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Colloidal Phosphorus in Surface Runoff and Water Extracts from Semiarid Soils of the Western United States

  1. Benjamin L. Turner *,
  2. Mary A. Kay and
  3. Dale T. Westermann
  1. USDA-ARS, Northwest Irrigation and Soils Research Laboratory, 3793 N. 3600 E., Kimberly, ID 83341


Colloidal particles in runoff may have an important role in P transfer from soils to waterbodies, but remain poorly understood. We investigated colloidal molybdate-reactive phosphorus (MRP) in surface runoff and water extracts of calcareous arable soils from the semiarid western United States. Colloidal MRP was determined by ultrafiltration and operationally defined as MRP associated with particles between 1 μm and 1 nm diameter, although a smaller pore-size filter (0.3 nm) was used to define the lower size limit of colloids in water extracts. In surface runoff from three calcareous soils generated by simulated sprinkler irrigation, colloidal MRP concentrations ranged between 0.16 and 3.07 μM, constituting between 11 and 56% of the MRP in the <1-μm fraction. Concentrations were strongly correlated with agronomic and environmental soil-test P concentrations for individual soils. Water extracts of a range of similar soils contained two size fractions of colloidal MRP: a larger fraction (1.0–0.2 μm) probably associated with fine clays, and a smaller fraction (3–0.3 nm) probably associated with Ca–phosphate minerals. Colloidal MRP was solubilized in the acidic medium of the colorimetric detection procedure, suggesting that a fraction of the filterable MRP in runoff from calcareous soils may not be as readily bioavailable as free phosphate in waterbodies. Our results suggest that colloidal MRP is an important but poorly understood component of P transfer in runoff from calcareous western U.S. soils and should be given greater consideration in mechanistic studies of the P transfer process.

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