Changes in soil acidity and the size and activity of the microbial biomass in response to the addition of sugar mill wastes

Abstract

The effects of additions of three wastes from sugar mills on the properties of two acid soils were investigated. The wastes used were boiler ash and filter cake from a conventional mill and fly ash from a new mill, where filter cake is burnt. Additions of each of the wastes raised soil pH, reduced concentrations of exchangeable Al and total and monomeric Al in soil solution and increased maize yields in a pot experiment. Microbial biomass C and the percentage of organic C present as microbial biomass were decreased by additions of wastes but the effect was less marked at the higher rate of addition. Although basal respiration was decreased or unaffected by the lower rate of addition of wastes it was increased by the higher rate. The metabolic quotient increased in the order: control<filter cake<fly ash<boiler ash and values were greater at the higher rate of addition. It was suggested that the pH increase induced by wastes resulted in a smaller, more bacterially dominated, community with a higher respiratory rate. The activities of arylsulphatase, acid and alkaline phosphatase were all increased by waste additions. Concentrations of exchangeable NH4+ and NO3- accumulated during incubation and arginine ammonification rate were all less at the higher rate of addition of each of the wastes. This was attributed to microbial immobilization of mineral N and arginine due to the wide ON ratio of light fraction organic matter present in the study soil. It was concluded that all three waste materials were effective liming materials but their effect on soil microbial activity was complex.