SUMMARYNumber of nodules and leghaemoglobin content of nodules increased with increasing Zn application up to 7·5 μg/g soil. Dry-matter yield and N fixation increased with Zn up to 10 μg/g soil. Both nodulation and N fixation decreased at higher levels. Soil N content showed an initial depletion but increased during the late season. Critical lower and upper levels for maximum N fixation were 1·75–2·5, and 10–14 μg of DTPA extractable Zn/g soil, respectively. In the present studies 5–10 μg Zn/g soil was sufficient for maximum N fixation in chickpea.
Laboratory studies were conducted to characterize Zn adsorption in some soils varying in texture using a wide range of equilibrating Zn concentrations. The fit of adsorption data was tested both in Freundlich and Langmuir isotherms. The Langmuir isotherm was resolved into two linear portions. Binding energy coefficients (K) were higher and adsorption maxima (b) were lower in part I than part II of the curves for both soils. b values were higher and K values were lower for loam than sand. The Freundlich isotherm was also resolved into three distinct portions having different K and n values. The adsorption pattern at different concentrations indicated differential bonding energies for adsorbed Zn, and occurrence of precipitation reactions along with adsorption in these soils, even before saturation of cation exchange capacity (CEC). The results showed the limitations in describing Zn adsorption through Langmuir or Freundlich adsorption isotherms in such soils of pH ⩾ 8 at 6 ppm Zn concentrations due to precipitation. Zinc adsorption exceeded CEC at equilibrating Zn concentration of 10−2M. The soil texture did not affect the nature of adsorption curves but more Zn was adsorbed by loam than sand showing the effect of the number of adsorption sites.
Influence of genetic variability on Zn response in recently improved corn (Zea mays L.) genotypes has not been investigated. Therefore, eight genotypes representing indigenous, hybrid, and composite varieties, were evaluated in the greenhouse using a Zn deficient loamy sand soil (Typic Torripsamments) at 0 (original soil), 5, and 10 ppm Zn applied to the soil. Zinc deficiency symptoms, dry matter yield response (6 weeks after sowing), Zn concentration, and P/Zn and Fe/Zn ratios were the criteria for evaluation. There was considerable variation among the eight genotypes in the severity of Zn deficiency symptoms, growth depression, Zn concentration, P/Zn, and Fe/Zn ratios under Zn stress conditions. The depression in shoot and root dry matter yield under Zn stress conditions ranged from 78 to 95 and 72 to 94%, respectively, as compared to 5 ppm supplemental Zn level. Zinc concentration in different genotypes under Zn stress condition varied from 7.4 to 20.5 ppm. The differential response among the genotypes was found to be associated with their capability to exploit soil Zn and/or translocate it to the shoot. The contribution of P/Zn and/or Fe/Zn balance in tissues to the variability in Zn responses was not significant.
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