A greenhouse experiment was conducted to study the effects of Zn-fertilizer on soybean yield and to determine its critical level in some selected soils of Mazandaran province. The soils were collected from 20 sites of eastern portions of Mazandaran province. They contained a wide range of CaCO3 (5-34%) and DTPA extractable Zn of 0.52 to 2.9 mg/kg of soil. In a randomized complete block design of three replications, 0 and 20 mg/kg of Zn as zinc sulphate were applied to l0 kg of soil in each pot. Pershing variety of soybean was planted at the rate of 7 seeds/ pot and then thinned but to 3 plants after l0 days .The growth period was a duration of 8 weeks. The aerial parts of plants were harvested at the end of the growth period, while the dry matter, nutrient concentration and uptakes were taken as plant responses. Plant samples were analyzed for Zn. The results indicated that zinc sulphate application significantly increased the dry matter (from 16.6 to 19.7 g/pot). Zinc application also increased leaf Zn content and Zn uptake by 69.9 and l04 percent, respectively. Based on these pot experiments, zinc critical levels in these soils were 1.30 and 1.10 mg/kg applying Mitcherlich–Bray and Cate–Nelson methods respectively, and for obtaining 85% relative yields. A comparison between the values obtained shows that both methods provide similar results.
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