Relationship between soil phosphorus availability and phosphorus loss potential in runoff and drainage

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 (P(bc))-calcium chloride, extractable P (P(cc)), P extractable by iron-impregnated paper (P(Fe)), P extractable by anion exchange resin (P(ae)), and the degree of P saturation (P(sat)). 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 P(cc), P(ae), and P(bc) (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 P(bc), P(ae), and P(cc) was characterized by a change point; runoff soluble P from soils