Impact of dairy pond sludge application on phosphorus fractions in paddock soil

Waste stabilization ponds (WSP) used to treat dairy shed effluent hold significant loads of recoverable nutrients in the sludge. Land application of this pond sludge over grazing paddocks is a potential alternative to chemical fertilizer. Desludging of WSPs can further enhance the nutrient removal efficiency of the WSP and thereby improve the water quality of the WSP effluent and limit accumulation of phosphorus (P) in WSPs. The aim of this study was to investigate the changes in the nutrient content of soil after land application of pond sludge (PS) collected from WSP over grazing paddocks. In particular, P fractions (e.g. labile P as H2O-P and NaHCO3-P, and stable P as NaOH-P and HCl-P) were analysed in pond sludge applied soil collected from grazed paddocks. It was found that the application of PS significantly changed the composition of phosphorus in the soil compared with a control paddock that was not treated with PS. The inorganic P in the paddocks treated with pond sludge was 605 mg/kg, which was significantly higher than 67 mg/kg observed in the control paddock. Similarly, the soil treated with pond sludge exhibited increased P fractions of 245%, 36%, 47% and 3000% in terms of H2O-P, NaHCO3-P, NaOH-P and HCl-P, respectively, compared with the control paddock. The results of labile and stable P fractions in the soil samples show that PS could be a viable alternative to other forms of commercial fertilizers and a sustainable source of P for the dairy grazing paddocks.