The impact of three different rotation breaks (legume/grass pasture, alternate crops and
bare fallow) on components of the soil biota was evaluated at three sites (Tully, Ingham
and Mackay) in Queensland. The breaks had different effects on the soil biota, but each
produced a large increase (rotation response) in the growth and yield of the subsequent
sugarcane crop. All three breaks reduced the population density of the lesion nematode,
Pratylenchus zeae, the number of spores of Pachymetra and the numbers of ground
pearls (Margarodidae). Microbial biomass increased under the pasture break at Tully
and Ingham but not at Mackay. Microbial biomass levels under the crop and continuous
cane treatments were similar at all three sites. Pseudomonas spp. (gram negative
bacteria) increased significantly under the pasture break at all sites but rotation-induced
shifts in populations of other culturable groups of micro-organism were variable. There
was, however, a general decline in all microbial groups measured under the bare fallow
break at Ingham and Mackay. Fatty acid analysis of the soils indicated that gram
negative bacteria, fungi and mycorrhizal fungi were present in significantly higher
amounts under the pasture break at each site. Plant growth experiments in the
glasshouse showed that the bare fallow break was as effective as fumigation in
removing yield decline symptoms from the soil. In contrast, the pasture and cropped
soils gave a fumigation response, suggesting that yield decline pathogens were still
present in these soils. However, this fumigation response was additive to the rotation
response achieved by these treatments, indicating the possibility of further increases in
sugarcane growth following these breaks.