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.