In a glasshouse trial, wheat (Triticum aestivum cv. Gamenya) was grown in pots using a range of southwestern Australian soils to which zinc (Zn) fertilizer was applied. Before wheat was planted, soil samples were collected to measure DTPA‐extractable Zn, which was related to the total dry matter production (straw and grain) of the wheat plants. The critical levels of DTPA extractable zinc (ZnD), determined for the maximum dry matter production of wheat, were found to vary among the 36 responsive soil types. Six soils were zinc‐adequate, and grain yield did not respond to zinc fertilizer. Consequently, for these soils the ZnD values were found to be the same or above the measured DTPA soil‐extractable Zn where no Zn fertilizer had been applied. ZnD values were related to (i) pH (1:5 w/v 0.01 mol/L CaCl2) of the soil (r2 = 0.74); (ii) clay content of the soils, which explained about 60% of the variation; and (iii) organic carbon content (%) of the soil. The equation selected in a stepwise linear regression was: * DTPA critical Zn level = 0.04 + 0.019 pH + 0.003 clay(%) + 0.004 organic carbon (%) (n = 36, r2 = 0.878).
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R. F. Brennan (1992) studied this question.