Final report Improving the cation retention capacity of cane-growing soils using high activity clays
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The Australian sugar industry largely relies on tropical soils that have low cation exchange capacities (CEC) and are prone to becoming deficient in Ca, Mg and K without appropriate management. Adding bentonite is an option for increasing the CEC, water holding capacity and fertility of these soils. This research project investigated if bentonite treatments could indeed be used to improve the fertility of low CEC sugar producing soils and enhance commercial cane yield. Two field trials conducted on old sugarcane producing soils in the Innisfail region in the Wet Tropics of far north Queensland over the 2006, 2007 and 2008 growing seasons showed that at rates of 10 - 30 t/ha banded additions of natural sodium bentonite improved soil properties and significantly raised sugarcane yields at final harvest. The best results were achieved with 20 and 30 t/ha rates of bentonite addition, in which cane yield was increased by up to 39.6 % in comparison to an untreated control. The main mechanisms responsible for the yield increase were found to be higher plant available water content (PAWC) and increased nutrient cation availability, which led to improved canopy development, greater radiation interception and overall enhanced growth and increased biomass accumulation within stalks. The results of the field trials were supported by five individual glasshouse trials that showed that various bentonite treatments could effectively be used to enhance soil CEC, nutrient cation levels and PAWC to bring about significant yield increases on a variety of low CEC soil types. Additional important information yielded by the glasshouse trials included the discovery that rates of above 80 t/ha bentonite had a detrimental impact on soil structure leading to reduced yields. Furthermore, the effect of increased PAWC as a result of bentonite treatment on yield was found to be much stronger than initially anticipated. An economic analysis assessing the feasibility of using bentonite treatments to improve soil fertility and increase cane yields has not yet been finalized. Preliminary results of this analysis suggest that bentonite treatment can indeed be an economically feasible option for increasing production and profitability in the long term in a permanent bed system under precision agriculture. Using an example from the Wangan trial where the cane yield in the 30 t/ha treatment was 97 t/ha compared to 75 t/ha in the control: assuming a sugar price of $480/t and a CCS of 12.5, the return per hectare for the 30 t/ha treatment is $772 higher than that of the control. At a bentonite cost of $336 per tonne, the cost of a banded application at a rate of 30 t/ha is $3360. Based on the increased return of $772 per hectare, the investment in the bentonite product would be paid off after five seasons. However, caution must be applied, as due to the high product cost of bentonite, the technique is economically unfeasible both in the short term and in a conventional farm system were the ground is reworked after three to four seasons.