BENTONITE TREATMENTS CAN IMPROVE THE NUTRIENT AND WATER HOLDING CAPACITY OF SUGARCANE SOILS IN THE WET TROPICS

THE Queensland 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 and improving the fertility of these soils. In a light textured Red Kandosol and a medium textured Red Ferrosol the basic CEC at soil pH was increased significantly by adding bentonite. The increases in surface charge are permanent and the correlation between the amount of bentonite applied and the change in CEC was linear. Significant increases in CEC of >1.0 cmol(+)/kg were achieved in both soil types at application rates of 20 t/ha and CECs >4.0 cmol(+)/kg were achieved at application rates of 50 t/ha. Bentonite at rates as low as 10 t/ha significantly increased the concentration and retention of applied nutrient cations. At application rates of 20 t/ha and above, natural sodium bentonite additions complemented by conventional nutrient management effectively maintained adequate levels of exchangeable Ca, Mg and K in soils under sugarcane over a full growing season (62 weeks), while nutrients in the untreated controls became deficient through leaching after just four weeks. Bentonite also increased the plant available water (PAW) content of soils. At an application rate of 30 t/ha the PAW content of a Kandosol was increased by 22%. Overall, the addition of natural bentonites at rates of 20 to 50 t/ha seems an effective means of permanently improving the availability of nutrients and PAW in low fertility sugarcane soils.