Degree of phosphorus saturation thresholds in manure-amended soils of Alberta

The risk of P losses from agricultural land to surface and ground water generally increases as the degree of soil P saturation increases. A single-point soil P sorption index (PSI) was validated with adsorption isotherm data for determination of the P sorption status of Alberta soils. Soil P thresholds (change points) were then examined for two agricultural soils after eight annual applications of different rates of cattle manure and for three agricultural soils after one application of different rates of cattle manure. Linear relationships were found between soil-test P (STP) levels up to 1000 mg kg(-1) and desorbed P in the five Alberta soils. Weak linear relationships were also found between STP and runoff dissolved reactive phosphorus (DRP) in three of these soils. Change points for the degree of P saturation (DPS) were detected in four of the five soils at 3 to 44% for water-extractable P (WEP) and at 11 to 51% for CaCl2-extractable P (CaCl2-P). Change points were not found for DPS or runoff DRP, Overall DPS thresholds for the five soils combined were 27% for WEP and 44% for CaCl2-P at a critical desorbable-P value of I mg L-1. The corresponding STP levels (44 mg kg(-1) for WEP and 71 mg kg(-1) for CaCl2-P) are similar to agronomic thresholds for crops grown on Alberta soils. Soluble P losses in overland How and leaching may be greater in soils with DPS values that exceed these thresholds than in soils with lower DPS values.