Responses in cotton yields to nitrogen and phosphorus fertilisers in the Emerald Irrigation Area, central Queensland

Abstract

The poor growth of cotton in the Emerald Irrigation Area of central Queensland was investigated in fertiliser experiments from 1978 to 1980. The need for large amounts of phosphorus (P) and nitrogen (N) fertilisers was demonstrated. In a glasshouse nutrient-omission experiment, N, P, potassium, sulfur (S), zinc, copper, manganese, boron (B) and molybdenum were tested for effects on cotton growth on the soil surface fraction (low in P and S) of a black cracking clay at Emerald. Plant growth indicated that P, N and S were extremely deficient. Other nutrients, except B, were marginally deficient. Slow growth was reflected in low shoot concentrations of N, P and S. In subsequent field experiments at 5 sites on 3 different soils, yield responses were only obtained to applications of P and N. Phosphorus at a rate of 40 kg/ha was required on a shallow, basaltic, dark cracking clay (Bug) soil. Yield responses to P were not obtained on an alluvial, dark cracking clay (AUg) or on a deep, basaltic, dark cracking clay (TbUg), but soil P concentrations were generally higher at these sites. For high value cotton crops where soil P analyses are low (i.e. <12 mg/kg bicarbonate-extractable P at 0-10 cm), high P fertiliser rates are recommended to maintain the soil P status. Nitrogen at a rate of 120 kg/ha produced heaviest yields on the Bug soil, but there were symptoms of N deficiency in plants early in the season. Irrigation frequency treatments produced no significant differences in yield.