Demonstrating the relationship between soil phosphorus measures and phosphorus solubility: Implications for Ohio phosphorus risk assessment tools

Approximately 80% of the land area draining into the western Lake Erie Basin is in Ohio, much of which is agricultural. Therefore, the potential for agricultural phosphorus (P) loading from Ohio is a concern for the water quality of western basin rivers, embayments and open water. This work demonstrates soil P relationships across Ohio soils and its implications for potential revisions of Ohio P risk assessment tools. The objectives were, using a selection of soils representative of soils across Ohio, (i) to determine if soil survey classification (series) could be an indicator of hydrous oxide content and, (ii) to determine if agronomic soil test P (STP) Mehlich 3 (M3-P), Bray P, or alternative measures of soil P saturation (Psat) were comparable to oxalate Psat for predicting P solubility, and therefore, useful for Ohio P risk assessment tools. Results showed no significant difference (P > 0.01) in soil hydrous oxide content when grouped by soil series. However, significant (P < 0.01) inflection points, reflecting a rapid increase in P solubility, were identified for oxalate P saturation (11.8%), M3-P saturation (12.4%), M3-P extractable P (181 mg/kg), and Bray-P extractable P (122 mg/kg). This suggests a separate pre- and post-inflection point consideration of STP may be appropriate for revised Ohio P risk assessment tools to better reflect increased post-inflection offsite P transport risk and thereby be sufficiently protective of water quality. Identifying fields with high offsite P transport risk is critical to implementing management decisions to reduce P transport risk to receiving waters including Lake Erie.