Regenerating a soil food web capable of improving soil health and reducing losses from soil borne pests and pathogens of sugarcane : final report 2014/004
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When the Sugar Yield Decline Joint Venture concluded in 2006, its main legacy was a new sugarcane farming system based on legume breaks, minimum tillage, controlled traffic and trash retention. Many economic benefits were obtained when the new system was adopted and soil physical and chemical properties also improved. However, the main biological improvements were temporary reductions in the number of nematode pests and cane-specific pathogens following the rotation crop. The aims of this project were to determine whether the soil biology had improved after this system had been in place for several crop cycles; assess the suppressiveness of the soils to nematode pests; identify the antagonists involved; and understand the effects of management on nematode suppression. Observations in 12 fields that had been farmed using the SYDJV system for 7-12 years indicated that most of the roots were unhealthy and that the nematode community was dominated by plantparasitic nematodes. The soil just below the trash blanket was much healthier as roots were functional, carbon levels were high and populations of plant-parasitic nematodes were being suppressed by their natural enemies. Previous work had shown that nematode-trapping fungi and predatory nematodes were important suppressive agents in sugarcane soils but studies undertaken in this project showed that predatory mites such as Protogamasellus mica and bacteria in the genus Pasteuria could also contribute. Evidence was obtained to indicate that trash blanketing enhanced suppressiveness to nematode pests whereas tillage and nitrogen inputs were detrimental. However, the main reason sugarcane soils had lost the capacity to regulate nematode populations was that their carbon contents were 50-80% below the levels attainable in a given soil type and environment. Attempts to increase suppressiveness with organic amendments were sometimes successful but the benefits obtained depended on the composition of the amendment, application rate and the method of application.