Effect of rotation breaks and organic matter amendments on the capacity of soils to develop biological suppression towards soil organisms associated with yield decline of sugarcane

A plant bioassay was developed to test the capacity of soil to suppress the activity of detrimental soil organisms associated with yield decline (YD) of sugarcane. The bioassay utilised the diseased roots of sugarcane plants growing in soil that had been under continuous sugarcane monoculture for more than 20 years, as the source of soil organisms associated with YD. Single-eye sugarcane setts were planted into pots of fumigated sand containing 2% (w/w) diseased roots and 10% (w/w) of the test soil. Suppression was measured as the capacity of the added test soil to block the detrimental effect of soil organisms associated with YD on plant growth. The bioassay indicated that a soil that had been under a pasture break for 7 years had increased biological suppression towards soil organisms associated with YD compared to a soil that had been under continuous sugarcane. There was little difference in suppression between sugarcane soils that had been under a soybean break for 1 year, a cropped soil that had never grown sugarcane and the soil that had been under continuous sugarcane. In contrast, a rainforest soil was found to have less suppression than the continuous sugarcane soil. Incorporation of organic amendments into a sugarcane soil (including sawdust, cane trash, grass hay, lucerne hay, feedlot manure, poultry manure, chitin and mill mud) initially increased fungal and bacterial populations, microbial activity (FDA hydrolysis) and microbial biomass. Plant bioassay tests of the amended soils 1, 7 and 12 months after the incorporation of the amendments indicated that the amendments generally had only a minor effect on the soils capacity to suppress soil organisms associated with YD.