Zinc availability to young wheat plants in Darling Downs black earths

A pot experiment in which wheat cv. Timgalen was grown in a glasshouse for 6 weeks clearly established that some of the 12 Darling Downs black earths investigated were zinc deficient. Rates of application between 1.2 and 3.7 kg zinc ha-1 (as ZnSO4.7H2O) were sufficient to achieve maximum or near-maximum dry matter yield of tops on the deficient soils. Plant zinc concentration and total zinc uptake in the tops increased in all soils with increasing rates of zinc application. The critical zinc concentration in the wheat tops associated with 90% of maximum dry matter yield was 20 �g g-1. Three extraction methods were used to determine the initial available zinc status of the soils. All methods gave lower extractable zinc values in the acid (pH <7 . 0 in 0.01 M calcium chloride) black earths (Mywybilla soils) than in the alkaline black earths. Critical extractable zinc values were much lower in the acid black earths. EDTA-ammonium carbonate (pH 8.6) more effectively separated zinc-deficient and marginally deficient from adequately supplied alkaline black earths than either EDTA-ammonium acetate (pH 7.0) or DTPA (pH 7.3). The critical extractable zinc value for the alkaline black earths with EDTA-ammonium carbonate was 0.61 �g g-1.