DEEP PLACEMENT OF ORGANIC AMENDMENTS IN DENSE SODIC SUBSOILS: EFFECTS ON SUGARCANE ROOT GROWTH, SOIL CARBON LEVELS AND SOIL BIOLOGICAL PROPERTIES

DUPLEX SOILS WITH low yield potential are relatively common in the Australian sugar industry. Since some of these soils have dense sodic subsoils that drain poorly and limit root growth, efforts are being made to improve their productivity. A grower group in the Maryborough region established a field trial in 2011 to test the effect of adding organic matter and found that when the soil was deep-ripped and compost was slotted into the subsoil at depths of 30?40 cm, the ameliorant increased sugarcane yields in two of the following five years. Samples collected after the fourth ratoon crop was harvested in 2016 showed that soil pH, total C, labile C, total N, microbial activity and the number of free-living nematodes were all significantly higher in the slot where the amendment had been applied. However, despite these improvements in soil chemistry and biology, root function in the slot treated with compost (measured as root biomass and various morphological parameters) was no better than the non-treated zone. In contrast, samples collected from two trials where organic materials had been applied to the subsoil 6 and 13 months previously showed significant improvements in root biomass and parameters such as root length, root surface area and the number of tips and forks. Also, numbers of root-lesion nematode (Pratylenchus zeae) were significantly reduced by mill mud in one of the trials. These results indicate that subsoil ameliorants initially improve root biomass and function and sometimes reduce pathogen loads. However, results from the trial where compost was applied five years previously suggest that the root health benefits provided by such ameliorants may decline with time.