Fertilizer source and soil properties affect soil test phosphorus and relationships with phosphorus losses in snowmelt runoff

Plant-available soil extractable P is used for making P fertilizer recommendations and can be helpful in assessing P loss potential in surface runoff water. Current efforts to relate soil test P (STP) to soluble P concentration in snowmelt surface runoff in the Canadian Prairie Pothole Region do not account for the relative solubility of the P source used nor crop productivity as influenced by the properties of the receiving soil. These factors also potentially influence the utility of STP as an indicator of runoff P losses. In this study, the relationship between STP (modified Kelowna extraction) and P concentration in simulated snowmelt runoff was evaluated across eight inorganic P fertilizer sources applied for 3 years at three sites of variable topography and soil properties in south-central Saskatchewan, Canada. Over 3 years, lower solubility P fertilizer sources had lower P concentrations in runoff that were predicted by assessment of soil residual P level in the fall. Phosphorus fertilizer applications, landscape positions, and crops grown that gave rise to higher crop P uptake and removal over time tended to lead to lower P losses in snowmelt runoff. In contrast, soil conditions that limited crop P uptake, including elevated salinity and low soil moisture, promoted P losses in surface runoff. These influences were also predicted by STP content, demonstrating the utility of the STP measure.