MINERAL-NUTRITION OF CASSAVA (MANIHOT-ESCULENTA CRANTZ) GROWN IN REPLACED SOIL AFTER BAUXITE MINING AT WEIPA, QUEENSLAND

The yield potential of cassava with optimal mineral nutrition was eval uated in a lateritic red earth that was replaced after bauxite mining at Weipa, Queensland. There were 9 field experiments. In 8 separate ex periments, 5 rates each of nitrogen (N), potassium (K), magnesium (Mg) , sulfur, copper, zinc (Zn), boron or molybdenum fertilisers were band ed into the soil. In the phosphorus (P) experiment, triple superphosph ate and rock phosphate were compared, each with 5 rates of P banded, b roadcast or spot-placed into the soil. After 2 wet seasons (66 weeks a fter planting), maximum tuber yields were produced by the banded appli cation of 200 kg P/ha as triple superphosphate, 20 kg Mg/ha and 8 kg Z n/ha. With rock phosphate, only the broadcast placement produced an in crease in the yield of cassava, maximum yields being achieved with 400 kg P/ha. In addition, there was evidence that 100 kg N/ha and 300 kg K/ha were needed for maximum yields. Therefore, based on the results o f these individual experiments over 2 seasons, 100 kg N/ha, 200 kg P/h a as triple superphosphate or 400 kg P/ha as rock phosphate, 300 kg K/ ha, 20 kg Mg/ha and 8 kg Zn/ha are the fertiliser recommendations for cassava grown in replaced soil at Weipa. In addition, early Zn deficie ncy symptoms (not related to any applied fertilisers) may necessitate a foliar spray of 4 kg Zn/ha as well as the soil-applied Zn fertiliser . However, the use of dolomite at 80 kg Mg/ha may have decreased the t uber yields and/or increased the requirements for certain soil-applied fertilisers. An average yield of 26.0 t/ha of tubers (fresh weight) w as obtained with a 51-week growing season, and the recommended rates o f fertilisers. This yield was reasonable when compared with 32 t/ha of fresh tubers predicted by a growth model for cassava, grown in North Queensland for 52 weeks without irrigation.