Influence of soil phosphorus status, texture, pH and metal content on the efficacy of amendments to pig slurry in reducing phosphorus losses

Cost-effective strategies for using chemically amended organic fertilizers need to be developed to minimize nutrient losses in surface and groundwater. Coupling specific soil physical and chemical characteristics with amendment type could increase their effectiveness. This study investigated how water-extractable phosphorus (P) was affected by chemical amendments added to pig slurry and how this effect varied with soil properties. A 3-month incubation study was conducted on 18 different mineral soils, stored at 10 degrees C and 75% humidity and treated with unamended and amended slurry which was incorporated at a rate equivalent to 19kg total P (TP)/ha. The amendments examined were commercial-grade liquid alum, applied at a rate of 0.88:1 [Al:TP], and commercial-grade liquid poly-aluminium chloride (PAC), applied at a rate of 0.72:1 [Al:TP]. These amendments were previously identified by the authors as being effective in reducing incidental losses of P. The efficacy of the amendments varied with the soil test P, the degree of P saturation (DPS) and the Mehlich aluminium, iron and calcium, but not soil texture. Chemical amendments were most effective in soils with DPS over approximately 20%. Due to their high cost, the incorporation of amendments into existing management practices can only be justified as part of a holistic management plan where soils have high DPS.