Response of ‘Sweet Spanish’ Onion to Soil-applied Zinc1

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Abstract
The effects of soil applications of Zn as ZnSO4 at 2, 4, 6, and 8 ppm and as ZnNa2-EDTA (disodium zinc ethylenediamine tetraacetate dihydrate) at 0.5, 1.0, 1.5 and 2.0 ppm on Allium cepa L. cv. Sweet Spanish dry matter and mineral composition were investigated in the greenhouse. Soil used was a sandy loam with pH 7.6, cation exchange capacity of 4.2 meq/100 g and available Zn of 0.63 ppm. Total dry matter (TD) increased with Zn up to 6 ppm as ZnSO4 but decreased significantly at 8 ppm, indicating Zn toxicity 108 days after planting. TD, however, was less than the control at all levels of Zn as ZnNa2-EDTA tested, and 1.5 and 2.0 ppm killed plants 87 days after they were sowed; that also suggested serious Zn toxicity. Zinc concentrations in all plant parts sampled increased with Zn applications. Leaf Zn concentrations correlated significantly with bulb dry matter. For correlations with bulb dry matter, leaf tissue determined Zn status of bulb onion better than did pseudostem or leaf plus pseudostem. Zinc distribution was influenced by Zn source. For the ZnSO4 source, bulb Zn was higher than leaf Zn; the reverse was true for the ZnNa2-EDTA source, at both 69 and 108 days after sowing. Zinc as ZnNa2-EDTA significantly increased Na concentrations in bulbs. Zn at high rates depressed leaf tissue concentrations of Fe, Cu, Mn, Ca, and Mg. NPK increased tissue content of Zn, P, Fe, Cu, Mn, Ca, Mg, and Na in the control plots in which NPK was supplied.

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