Populations of free-living nematodes (FLN) increased and plant-parasitic nematodes
(PPN) decreased at Tully, Ingham, Burdekin, Mackay and Bundaberg when a
grass/legume pasture or legume crop was grown for 30–42 months before planting
sugarcane. At other sites, shorter rotations also had significant effects on nematode
communities. In north Queensland, a single soybean crop increased pre-plant
populations of FLN and reduced populations of pest species such as Pratylenchus zeae
by 44–89% compared to similar fields that had grown sugarcane. In a replicated
experiment at Bundaberg, a soybean/peanut rotation increased numbers of FLN and
reduced populations of two plant parasites (P. zeae and Rotylenchulus parvus) by 75%
and 93% respectively. When sugarcane was planted following a legume, the ratio of
FLN to PPN was generally greater than 20, whereas this ratio was often less than 2 in a
plough-out/replant situation. It is possible that this change in the balance between
beneficial and pest nematodes partly explains the improved early shoot growth and
increased cane yields that are consistently obtained when sugarcane is planted following
a legume. FLN are beneficial because they enhance nutrient cycling by grazing on soil
microorganisms. They also help maintain populations of the predators that feed on
nematodes. In contrast, PPN have a negative impact on sugarcane growth because they
destroy fine roots, limit root elongation and debilitate root systems.