EFFECTS OF THE APPLICATION OF MILL MUD/MUD ASH ON SOIL PROPERTIES AND AVAILABLE SOIL NITROGEN AND PHOSPHORUS

MILL MUD and mud ash are valuable sources of nutrients (nitrogen, phosphorus, potassium) as well as having high contents of organic carbon. Pot experiments and an incubation study were carried out using three cane soils of differing textures to determine the rate of release of nitrogen (N) and phosphorus (P) from the products for uptake by sugarcane, and to determine the effects of mill mud on key soil chemical and physical properties. Approximately 10% of the total N applied in the mill by-products was recovered in sugarcane tops grown for 120–142 days, and using the availability of urea-N as a benchmark, it was estimated that a further 25% of the total N applied in the mill by-products would be available for crop uptake in the first season. Phosphorus availability in the products could be measured using BSES-P, and an equation was developed for calculating soil BSES-P after amendment with the mill by-products. Application rates in excess of 100 wet tonnes/ha amended soil were required for measurable changes in soil cation exchange capacity and aggregate stability, and these changes only occurred in the light-textured (3% clay) soil. However, these application rates resulted in unacceptably high concentrations of dissolved inorganic P in the soil solution that posed a risk to water quality.