Phosphorus Storage on Effluent Irrigated Land

Land disposal of secondary treated sewage effluent is being widely adopted in Australia as an alternative to tertiary treatment. At the Cleveland Waste Water Pollution Control Works, effluent has been applied to a Hapludalf at high rates (3000 mm yr ¹) for more than 20 yr, adding an estimated 4.5 t P ha ¹ to the soil. Thus, this site provides an opportunity to assess the retention of P under conditions typical of southeast Queensland. Laboratory chemical fractionation and P sorption studies of effluent irrigated and control sites were undertaken to determine the amount and forms of P retained within this soil and its capacity to retain further P. Organic material (peat) which had accumulated ou the surface of effluent irrigated plots, was also sampled and its P content determined. Accumulation of P was limited to the sandy surface soil, increasing total soil P content by ~ 700 kg ha ¹ , with this increase being approximately evenly distributed between inorganic and organic pools. The excessive irrigation rate used on this site, and the management adopted, has resulted in a marked podzolization of the surface horizon, apparent in reduced extractable Fe and AI concentrations. The removal of these P adsorbing materials would contribute, along with P saturation and organic ligand competition, to the much lower P sorption capacity of irrigated sites relative to the control. The largest P pool was found to be the peat layer, containing approximately 1500 kg P ha- ¹. On this site P has accumulated principally in organic forms, both within the soil and as peat on its surface. This finding throws into question the common practice of predicting disposal scheme life on the basis of P adsorption curves.