Estimating Phosphorus Saturation as an Indicator of Phosphorus Loss Risk in Acidic Soils of Assam, India

Abstract

Acid soils exhibit distinct phosphorus (P) dynamics compared with neutral or alkaline soils. When the P concentration in the soil solution exceeds a threshold, saturation occurs, increasing the risk of P loss through leaching or surface runoff. This study evaluated phosphorus saturation indices as indicators of P-loss risk in acid soils of Assam, India. Sixty soil samples were collected from three locations in Assam representing three distinct soil orders. Based on Pmax values, the soils were incubated with graded P concentrations and extracted using five extractants: ammonium oxalate, Mehlich-3, Mehlich-1, Bray-2, and Bray-1. The degree of phosphorus saturation (DPS) was subsequently determined. Kharupetia soils (Entisol) recorded the highest phosphorus adsorption maxima (Pmax), ranging from 769.23 to 1666.67 µg g⁻¹. Ammonium oxalate consistently extracted more P than the other extractants, and extractable P increased with P addition. DPS values increased significantly with increasing P concentrations (10–400% of Pmax). Surface and subsurface soils from Kharupetia showed the highest DPS ranges, with DPS(AO) reaching 0.26–292.86% in surface soils and 0.25–270.38% in subsurface soils. Threshold DPS values (>30%) varied by soil type and extractant. At 50–100% of Pmax, DPS(AO) exceeded the threshold, indicating a high risk of P loss, whereas the other extractants crossed the threshold only at 100–200% of Pmax. DPS(AO) showed the greatest sensitivity and was therefore the most suitable index for assessing phosphorus-loss potential. Overall, DPS sensitivity followed the order DPS(AO) > DPS(M3) ≈ DPS(M1) > DPS(B2) ≈ DPS(B1).