IMPACT OF FALLOW LENGTH, ORGANIC AMENDMENTS, BREAK CROPS AND TILLAGE ON SOIL BIOTA AND SUGARCANE GROWTH

FIELD experiments were established at Bundaberg (South Queensland) and Abergowrie (North Queensland) to quantify the impact of different durations of bare fallow and inputs of organic materials of differing quality (grass, legume or grass/legume mixtures) and origin (grown in situ or imported) on soil biota and growth and yield of sugarcane crops. Fallow lengths ranged from 1 month to 30 (Bundaberg) or 35 months (Abergowrie). Imported organic materials were added as finely chopped hays to add 10 t C/ha, while C inputs from crops grown in situ varied depending on crop frequency and seasonal conditions. The impacts of tillage prior to cane planting were also investigated for the in situ organic matter treatments at both sites, while soil fumigation was used to benchmark potential biological constraints in the bare fallow treatments. Significant differences between treatments were recorded in soil C, general soil biology and the incidence of known cane pathogens. All parameters declined with increasing length of bare fallow, although cane specific pathogens like Pratylenchus zeae declined more rapidly than general biological activity. Imported organic amendments caused only small changes in soil biology and chemistry, but in situ grass pastures were able to achieve soil C and biological activity similar to those found in soil in the cane rows. Long-term legume cropping generally resulted in less soil C and biological activity, especially when plots were tilled conventionally. There were significant differences in the resident soil microbial communities under cane, grass pastures and legume cropping. Despite the large changes in soil biota in the long bare fallows and in situ grass or legume treatments, growth responses of a subsequent plant cane crop were negligible at Abergowrie and relatively small at Bundaberg (10–30%). No treatment was able to match improvements achieved in fumigated ploughout/replant. The best crop responses were achieved from the in situ organic matter treatments, especially the grass pastures, with part of this response possibly due to slower re-establishment of pathogens like lesion nematode. Tillage after both grass pasture and legume cropping increased the rate of recovery of lesion nematode populations. Results have implications for management of residues and cane trash in the sugarcane cropping system. Future cropping systems need to maximise the soil health benefits of sugarcane cropping (perennial growth, regular returns of organic matter) by adopting complementary management strategies (short legume fallows, reduced tillage, optimal use of cane trash) that build on these strengths.