Potassium management for sugarcane on base saturated soils in northern New South Wales

AN industry-wide survey of the nutrient status of sugarcane crops in 2000–01 showed leaf potassium (K) levels in most samples from New South Wales were well below the critical value. There were no data to indicate the significance of this information for crop yield and future K management. In 2002, rates of K fertiliser experiments were established at two sites in the Broadwater Mill area on clay soils where exchangeable K was 0.13–0.14 cmol(+)/kg and nitric K was 0.78–1.04 cmol(+)/kg. Calcium (Ca) + magnesium (Mg) saturation was 91–95% of total bases. The lowest rates of applied K were the growers’ strategy of 39 and 42 kg K/ha. Treatments of 0 to an additional 300 kg K/ha were applied in 50 kg increments as row dressings. Five crops were harvested between 2003 and 2005. Leaf K% was always below the critical value in the standard treatment, and above the critical value for some of the higher rates of K in 2003 and 2005. There was no yield response to K fertiliser above the basal rate indicating supply of K from non-exchangeable soil reserves was adequate. Biomass K increased with rate of applied K, but the K balance was negative when less than 192 kg K/ha was applied. Leaf data suggested high levels of (Ca+Mg) in soil may be interfering with K uptake. The ratio of (Ca+Mg)/K >0.55 can be used to identify leaf samples from base saturated soils that may require interpretation using a reduced critical value in the range 0.75–0.9% K. Absence of yield responses showed that the crop does not need more K than is currently applied, but soil K status should be monitored to indicate when potential for yield response should be re-evaluated.