Phosphorus sorption capacities of calcareous soils receiving cattle manure applications for 25 years

Manure management is an important issue for cattle producers in areas where many intensive feedlots operate. Accumulation of phosphorus (P) in agricultural soils receiving repeated manure applications and transport of P from manure to ground and surface waters has been documented. This study investigates the P sorption capacities of soils amended with feedlot manure annually for up to 25 years. Non-irrigated plots at the Lethbridge Research Centre, Alberta, Canada, received 0, 30, 60. and 90 Mg manure (wet weight) ha(-1), and irrigated plots received 0, 60, 120, and 180 Mg ha(-1) annually for 25 years. The quantity of P sorbed to surface soils (0 to 15 cm) from solutions containing 0 to 40 mug P mL(-1) was determined, and sorption isotherm models were fitted to the Freundlich equation. The P sorption capacity of non-irrigated and irrigated soils declined with increasing soil available P concentrations, but the P sorption capacity of surface soils that received applications annually for up to 25 years was generally not more than 50% lower than the P sorption capacity of unamended soils. Soil organic matter content, pH, and surface chemistry were altered by manure application, and may explain why the P sorption capacity did not decline more as soil available P concentrations increased. Further investigations will be required to determine the fate of manure P not sorbed in manure-amended calcareous soils to promote manure management practices that are agronomically sound and protect water quality.