THE IMPACT OF MILL MUD AND COMPOST AMENDMENTS ON THE BIOLOGY OF SUGARCANE SOILS

GIVEN THE CENTRAL role of organic matter in improving soil physical, chemical and biological properties, some Australian canegrowers established replicated trials on their farms to determine whether locally-available organic wastes could be used to improve the biological health of their soils. In three of these trials, various chemical and biological parameters were assessed 4?28 months after composts made from household waste and other organic materials were applied at 3?6 t dry matter/ha. However, results were disappointing because compost had little impact on soil carbon levels, microbial activity or the composition of the nematode community. In contrast, applications of mill mud provided benefits in one of these trials. Eighteen months after mill mud was applied to a soybean rotation crop, free-living nematode populations were higher than in the non-amended control. Also, soil carbon levels increased when mill mud was applied prior to planting sugarcane, while successive applications of mill mud increased soil microbial activity. In a fourth trial in which double-disc openers were used for three successive years to slot mill mud into furrows on either side of the cane row, soil carbon levels in the zone where the mill mud was placed were almost double the levels in the untreated zone; microbial activity and numbers of free-living nematodes increased significantly; numbers of plant-parasitic nematodes declined by about 67% and there was a profusion of fine roots in the area where the amendment had been placed. Given the variability in the results obtained with different amendments and application strategies, long-term trials are required to determine the best way of integrating organic amendments into the sugarcane farming system to improve soil biological health.