Phosphorus bioavailability in alum biosolids used in agriculture
Rigby, H. and Pritchard, D.L. Curtin University, Australia
(free)
Effluent discharged from wastewater treatment plants to inland water bodies has the potential to cause eutrophication unless the phosphorus (P) concentration is reduced by methods such as dosing with alum (Al2(SO4)3), to precipitate P and capture it in the solid sludge. As a consequence the sludge has a relatively high concentration of aluminium (Al) and P compared to sludge (or biosolids) produced without chemical treatment. Alum biosolids have not been widely used for crop production and there is little research that has examined the effect of the high concentration of Al on crop growth. The use of alum residuals from drinking water treatment, however, has indicated that the Al may contribute to a reduction in P uptake by plants. A field experiment was established to investigate the use of alum biosolids for cereal production to make better use of the nutrients contained in the sludge, which is typically landfilled. Alum biosolids (7.5% Al, 3.7% P) were applied at six rates (0, 3.4, 6.7, 10.1, 13.5 and 20.1 Mg DS ha-1) on a P deficient sand. In addition, one rate of inorganic fertiliser at district practice containing 20 kg P ha-1 was applied. The inorganic fertiliser treatment yielded 44% higher than the nil fertiliser treatment at final grain harvest, demonstrating the requirement for P on this site for crop production. The addition of alum biosolids at rates of up to 20.1 Mg DS ha-1 did not increase grain yield (>0.05), with the exception of the 13.5 Mg DS ha-1 treatment, with yields similar to the fertiliser control.
Plant shoot tissue analysis at the tillering stage of the wheat (61 days after sowing), indicated that plants sown in alum biosolids-amended soil were P deficient, whereas P was adequate in the inorganic fertiliser treatment. There was no evidence of any other nutrient deficiency in plant shoot samples besides P. Therefore, it is suggested that on this P deficient soil, the ability of alum biosolids to provide P for early cereal growth was poor. Soil analysis indicated that P may become slowly available in the alum biosolids treatments and therefore the residual effect of P in alum biosolids on plant growth in the second season will be investigated.
