Soil Phosphorus Status and Environmental Risk

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  • 1 Univ. of California, Dept. of Vegetable Crops, Davis, CA 95616
  • | 2 Univ. of California, Dept. of Vegetable Crops

Heavy P fertilization in the Salinas Valley of California has increased soil P concentration to levels of environmental concern. To determine the correlation of various soil test procedures with P pollution potential from agricultural land in this region, soil was collected from 30 fields, most in long-term vegetable rotations. Soils were analyzed for bicarbonate-extractable P (Pbc), calcium chloride-extractable P (Pcc), bio-available P (Pba, by an anion-resin membrane technique), and %P saturation (Psat, by an enrichment technique). The soils were then exposed to a simulated irrigation event, and soluble P concentration in runoff determined. In a separate experiment the effect of cover cropping on sediment and soluble P concentration in runoff was investigated; containers of six soils were planted with oats (Horteum vulgare L.), and then compared to containers of fallow soil. Pcc, Pba and Psat were all highly correlated (r = 0.86, 0.89 and 0.90, respectively) with Pbc, which ranged from 15-177 mg·kg-1. Soluble P concentration in runoff was highly correlated with all measures of P status (r = 0.98, 0.93, 0.85 and 0.83 for Pcc, Pba, Psat and Pbc, respectively). These results suggest that while Pbc, the standard agronomic measure of soil P status, is a useful indicator of P pollution potential, Pcc (a simple laboratory procedure that could be adapted as an on-farm `quick test' technique) may be superior for that purpose. Across soils, cover cropping reduced soluble P concentration in run-off by 41%, and sediment in the runoff by 85%.

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