P.R. Johnstone

Relationship between Soil Phosphorus Availability and Phosphorus Loss Potential in Runoff and Drainage

Abstract Elevated soil phosphorus (P) content is common in the central coastal valleys of California, the result of decades of the intensive vegetable production. Undesirably high P concentration in surface water in this region stimulated interest in evaluating techniques to rank the potential for soil P loss to the environment. Phosphorus availability of 25 representative soils from fields in vegetable rotations were evaluated by the following techniques: bicarbonate‐extractable P (Pbc)–calcium chloride, extractable P (Pcc), P extractable by iron‐impregnated paper (PFe), P extractable by anion exchange resin (Pae), and the degree of P saturation (Psat). A column study was conducted in which these soils were evaluated for soluble P concentration in runoff and leachate from two simulated irrigation events. There were strong correlations among all measures of soil P availability (r=0.66–0.90). Runoff soluble P was most strongly correlated with Pcc, Pae, and Pbc (r=0.98, 0.93, and 0.91, or 0.98, 0.90, and 0.85 in the first and second irrigation, respectively). The relationship of runoff soluble P to Pbc, Pae, and Pcc was characterized by a change point; runoff soluble P from soils <50 mg kg−1 Pbc was minimal, whereas at higher Pbc runoff P reached levels of environmental concern. Leachate soluble P was also correlated with Pcc, Pae, and Pbc (r=0.84–0.99). Across soils, leachate soluble P averaged 1.4 mg L−1, compared to 0.11 mg L−1 for runoff P. We conclude that Pcc, Pae, and Pbc are useful tests to rank the potential for P loss in irrigation runoff or drainage. Given the relative complexity of the Pae technique, Pbc and Pcc appear to be the most practical soil tests for this purpose.