Sugar Mill Wastes Can Be Important Soil Amendments

The effects of additions of three wastes from sugar mills on the properties of an acid soil were investigated. The wastes used were boiler ash and filter cake from a conventional milling operation and fly ash from a new milling process, where filter cake is burnt in the boilers. Additions of each of the wastes at two sites raised soil pH and increased maize yields in a pot experiment; the effect being greater at the higher rate. Concentrations of exchangeable Al and total and monomeric Al in soil solution were reduced by additions of each of the wastes. Thus, the increase in pH caused precipitation of potentially phytotoxic soluble and exchangeable Al as hydroxy-Al polymers. Waste additions increased the basal respiration rate and the activities of the soil enzymes arylsulphatase and acid phosphatase. Additions of filter cake increased the organic C content of the soil and the amount of N mineralised during incubation. It was concluded that all of the waste materials had a liming effect and at high rates they increased the microbial activity in the soil. In particular, filter cake was found to be a valuable organic amendment that should be recycled back onto sugarcane fields to increase their fertility and organic matter status.