Agronomic Issues with Alum-Sludge

A two year field experiment was established to investigate the use of alum sludge from wastewater treatment for cereal production. Alum sludge (5.0% N, 3.7% P, 7.5% Al) was 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, supplying up to 744 kg P ha-1 at the highest rate. In addition, one rate of inorganic fertiliser at district practice was applied, containing 72 kg N ha-1 and 20 kg P ha-1, this was reapplied in the second year. The inorganic fertiliser treatment yielded 44-58% higher than the nil fertiliser treatment at final grain harvest, demonstrating the requirement for applied N and P on this site for crop production. Alum sludge was an adequate source of nitrogen (N) for crop growth as indicated by plant tissue N content, and supplied sufficient residual N to meet crop requirements in the second year.However, grain yield in alum sludge treatments was reduced to 62% (year 1) and 69% (year 2) of the yield in the inorganic fertiliser treatment, though greater than the nil fertiliser treatment in both years. Plant shoot tissue analysis at 9 weeks after establishment at the tillering stage of development indicated that plants sown in alum sludge-amended soil and in the nil fertiliser treatment 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 sludge to provide P for plant production was limited in the two years after application