Long-Term P Balances Explain Differences in Soil P Stocks and Concentrations

Agricultural phosphorus (P) inputs are often unbalanced with P outputs, resulting in surpluses and deficits that can lead to undesirable soil P buildup or depletion. The degree to which soil total P and available P stocks and concentrations respond to P balances determines the long-term sustainability of P management practices. This study characterized the relationships between soil P and P balances after 18 years of a P fertilizer rate experiment conducted at three locations in Ohio, USA. Phosphorus balances, calculated as P fertilizer inputs minus P removed in grain harvest, ranged from −403 kg ha−1 to +570 kg ha−1 and were primarily driven by fertilizer rates, while differences in corn and soybean yields and grain P concentrations were of secondary importance. Deep (0–90 cm) soil Mehlich-3P (M3-P) and total P (TP) underwent depletion under negative P balance and accumulation under positive P balance. Soil P differences were largest in the 0–5 cm depth, although differences in soil TP were observed for the full 90 cm sampling depth, indicating that assessments of long-term changes in P stocks should take into account subsoil P. The difference in M3-P stocks encompassed 21% of the differences in the soil TP stocks, indicating that the soil P changes were dominated by less available P forms not extracted by M3. The three locations showed subtle differences in the magnitude and depths of differences in M3-P concentrations and stocks, highlighting the site-specific nature of M3-P changes. Phosphorus balances were linearly related to soil P stocks, with the average slope estimate indicating each 1 kg ha−1 change in P balance was associated with a 0.87 kg ha−1 change in soil TP stock. Overall, responses of soil P stocks to the long-term P balances were close to expectation across the three experimental locations, demonstrating that P balance management is a vital tool for achieving nutrient management and water quality goals.