The effectiveness of zinc, as either zinc sulfate (ZnSO4.7H(2)O,22.4% Zn) or zinc oxide (ZnO; 80% Zn) applied to an acid sand or an alkaline sandy clay, at producing wheat shoots was compared in a glasshouse experiment using yield of 50-day-old wheat (Triticum aestivum L.) plants. The fertilisers were applied as fine powders and mixed through the soil. Both fertilisers were equally effective in the acid soil, but the oxide was about half as effective as the sulfate in the alkaline soil; about twice the amount of zinc as the oxide was required to produce the same yield as zinc added as the sulfate. The amount of zinc required to produce 90% of the maximum yield was 38 mu g Zn/pot for both sources of zinc in the acid soil, and 100 mu g Zn/pot for the sulfate source and 250 mu g Zn/pot for the oxide source for the alkaline soil. Critical zinc, which is the zinc concentration in the youngest emerged leaf that was related to 90% of the maximum yield of shoots, was about 13 mg/kg for both sources of zinc and both soils. Zinc oxide may be less effective at producing wheat shoots than zinc sulfate in alkaline soils of south-western Australia.