Degree of Phosphorus Saturation as Environmental Threshold for Acidic Soils of Eastern India

The establishment of a soil indicator is essential to predict the potential for phosphorus (P) movement from a nonpoint soil source to surface water bodies, causing environmental pollution. The present study was undertaken with two pedogenically different acidic soils (terai and lateritic) in eastern India to estimate their environmental threshold degree of P saturation (DPS) values as an index of the potential risk for P loss. Additionally, this study rigorously evaluates the feasibility of Bray-1 as a standardized extractant for environmental phosphorus loss risk assessment, analogous to the recognized effectiveness of Mehlich-1 across diverse locations. Both Bray-1 and Mehlich-1 methods are equally efficient in determining environmental P-threshold, and the change point DPS at which water-soluble P increased sharply with these extractants was around 20%. The DPS (%) calculated using the Bray-1 method (DPS_B-1) revealed that even a high agronomic soil test rating could not indicate the environmental risk. Based on a multiple linear regression model including soil properties like, clay content (%) and pH analyzed in most soil analysis laboratories along with Bray-1 P, was found to be an efficient method (DPS_est) in predicting change point DPS. Considering the change points, confidence intervals, and agronomic soil test values, three categories of DPS_B-1/ DPS_est < 20%, 20–40%, and > 40% were established as safe, warranting attention, and critical levels of P loss from acidic Indian soils, respectively.

Graphical Abstract