Soil variables for predicting potential phosphorus release in Swedish non-calcareous soils

The accumulation of P in agricultural soils due to fertilization has increased the risk of P losses from agricultural fields to surface waters. In risk assessment systems for P losses, both P release from soil to solution and transport mechanisms need to be considered. In this study, the overall objective was to identify soil variables for prediction of potential P release from soil to solution. Soils from nine sites of the Swedish long-term fertility experiment were used, each with four soil P levels. Phosphorus extractable with CaCl2 was used as an estimate of potential P release from soil to solution. Ammonium lactate– extractable phosphorus (P-AL) or NaHCO 3–extractable phosphorus (Olsen P) could not be used alone for prediction of potential P release since soils with high phosphorus sorption capacity (PSC) released less than soils with low PSC at the same soil test phosphorus (STP) level. Degree of phosphorus saturation (DPS) was calculated as Olsen P or P-AL as a percentage of PSC derived from P sorption isotherms or from Fe and Al extractable in ammonium oxalate. The CaCl2– extractable total phosphorus (CaCl2–TP) was exponentially related to these DPS values (r2 _ 0.79). The CaCl2–TP was also linearly related to ratios between Olsen P or P-AL and a single-point phosphorus sorption index (PSI; r2 _ 0.86). These ratios, which are easily determined and gave good correlations with CaCl2–TP, seemed to be the most useful estimates of potential P release for risk assessment systems.