Decrease in phosphorus concentrations when P fertiliser application is reduced or omitted from grazed pasture soils

Many intensively managed soils contain phosphorus (P) concentrations greater than required for optimum production. Soils with P concentrations in excess of the agronomic optimum can have unnecessary losses of P that can adversely affect water bodies. Reducing excessive soil-P concentrations is important for the economic and environmental sustainability of intensive agriculture, such as the Australian dairy industry. However, little is known of decreases in extractable soil-P concentrations when P fertiliser applications are reduced or omitted from soils with P concentrations and properties representative of intensive pasture grazing systems. Decreases in extractable P (calcium chloride (CaCl₂), Olsen and Colwell) were monitored for up to 4.5 years for six Australian grazed pasture soils (Red Ferrosol, Brown Kurosol, Grey Dermosol, Brown Dermosol, Podosol and Hydrosol) with contrasting textures and P-buffering indices (PBI). Sixteen treatments consisting of four initial extractable-P concentrations (P_init) paired with four ongoing P fertiliser rates (P_fert) were established for each of the six soils, except on an extremely low-PBI Podosol, where a range of P_init concentrations could not be established. The resultant decreases in P were larger with higher P_init concentration and lower rate of ongoing P_fert, except in the extremely low PBI Podosol where decreases in initially high CaCl₂-P concentrations were large irrespective of ongoing P_fert. There was a greater proportional decrease in the environmentally extractable P compared with agronomically extractable P, with mean decreases in CaCl₂-P of 57%, Olsen-P of 25%, and Colwell-P of 12%. The P_init concentrations, which were well above agronomic optimum, remained above this target. This study advances scientific knowledge of extractable soil-P concentrations when P fertiliser inputs are withheld or reduced from grazed pasture soils, and aids land and catchment managers in estimating likely changes over time.