ASSESSMENT OF ZINC AND IRON, ZINC ADSORPTION, AND EFFECTS OF ZINC AND IRON ON YIELDS AND GRAIN NUTRIENT CONCENTRATION OF TEFF AND BREAD WHEAT IN SOME SOILS OF TIGRAY, NORTHERN ETHIOPIA

Abstract

Zinc and iron are among the essential plant nutrients required in relatively small quantities for plant growth. Deficiencies of Zn and Fe have been reported in soils of Tigray, Ethiopia. The deficiencies of Zn and Fe in the soil system affect both plant growth and the nutrient contents in the grain of cereals for human consumption. This study was developed to (1) assess the status of Zn and Fe in leaves of teff and bread wheat in relation to soil contents and properties in agro-ecologies growing these crops (2) establish Zn adsorption characteristics and its quantitative relationship with selected soil properties (3) study the effects of Zn containing fertilizers, Fe chelate fertilizer types and supplemental Fe levels on yield and grain Zn and Fe concentrations of teff and bread wheat. Plant and soil status survey, laboratory and greenhouse experiments were conducted. The soil and leaf test results revealed that regardless of agro-ecological differences, Zn deficiency in soil and the crops was widespread. Of the total soil, teff and wheat samples 90, 98 and 89% were deficient in Zn, respectively. Available Zn in the soil ranged between 0.18 to 2.26 mg kg-1soil. Tissue Zn in teff ranged between 5 to 22 mg kg-1, while it was ranged 2 mg kg-1 in Tepid to cool sub moist mountains and plateau (SM2-5) agro-ecology to 42 mg kg-1in Cold to moist highlands (M2) for wheat. Only 5% of the total soil samples were deficient in Fe. Teff tissue Fe concentration ranged between 29.5 mg kg-1 in Hot to warm sub-moist lowland plains (SM1-3) agro-ecology to 868 mg kg-1 in M2 agro-ecology, while bread wheat Fe concentration ranged between 25 mg kg-1 in SM2-5 to 653 mg kg-1 in M2. The variability in Fe concentration in the crops could due to the combined characteristics effects of the agro-ecologies. Iron deficiency in teff and bread wheat plants was not observed in M2 agro-ecology, while deficiencies in teff in SM2-5 and SM1-3, and bread wheat in SM2-5 were observed. Zinc concentration of teff leaf significantly and positively correlated with total soil Zn while Zn concentration of bread wheat was not correlated with soil Zn. Both crops did not correlate with extractable or available Zn in the soil. Iron concentration of teff significantly and positively correlated with total Fe in soil but not with DTPA-Fe, while Fe concentration of bread wheat correlated significantly and xvi positively with total Fe in soil and DTPA-Fe. Zn adsorption conformed both to Freundlich adsorption (R2 = 0.936 to 0.998) and Langmuir adsorption (R2 = 0.938 to 0.980) isotherms. Soil pH, clay, calcium carbonate, soil organic carbon content and cation exchange capacity contributed to the adsorption of Zn in the studied soils. Zinc fertilization increased grain yields of teff (27%) and bread wheat (10-84%) in Zn deficient soils of Tigray but may not be promising strategy to increase Zn in the grains of these crops destined for humans. Application of Fe-EDDHA1 chelate at a rate of 2.5 kg Fe ha-1 improved teff grain yield (13.6%) and grain Fe content (18 to 71%). Though, there were significant improvements in bread wheat grain yield and grain Fe content by Fe application, the increments were small. It can be concluded that plant tissue analysis along with soil analysis can give a complete picture in the investigation of soil fertility assessment. The findings of Zn adsorption have implications to soil fertility management with respect to Zn availability and developing management strategies for high and low Zn adsorbing soils. Zinc application improved yields of teff and bread wheat but would not be a promising strategy to increase Zn concentration in the grains of these crops destined for human consumption. Iron chelates HBED and EDDHA1 improved both yield and Fe concentration of teff but the increments in bread wheat were small. Hence, agronomic management options that improve both yield and grain nutrient concentrations of the crops should be developed. However, more rigorous studies and field verifications of the current studies are recommended to draw sound conclusions.