Sorption of Zinc and Iron in Soils of Selected Areas in Southern Ethiopia

Abstract

Sorption is one of the most important chemical processes in soils that determine the quantity of plant nutrients retained on soil surfaces and, therefore, affect transport of nutrients and regulate the concentration of nutrients in soil solution. A study was conducted in soils of Zeway, Alage, Butajira, Hawassa Zuria, Halaba and Taba in southern Ethiopia with the objective of evaluating the extent of Zn and Fe sorption.  Surface soils (0-30cm depth) from the sites were collected, air dried and analyzed for particle size distribution, pH, EC, CEC, organic C, calcium carbonate equivalent, macro and micro nutrients using standard procedures.  A 0.5 g soil sample was placed in a 100 ml plastic bottle and equilibrated with 0.01 M CaCl₂ overnight using a soil/solution ratio of 1:50 to determine the sorption of Zn and Fe. Stock solutions of ZnSO₄ and FeSO₄ with concentrations of 0, 2, 5, 10, 15 and 30 mg L^-1Zn Fe respectively were mixed with the soil-0.01 MCaCl₂ solution and equilibrated for 24 hours. The suspension was filtered, and the concentrations of Zn and Fe in the clear extract solution were determined using Atomic Absorption Spectrophotometer. The Langmuir and Freundlich adsorption equations were applied to determine the sorption relationships. The results indicated that the soils differ in physicochemical characteristics as well as Zn and Fe sorption capacities. Additionally, sorptions of the two micronutrients were high in all soils, except that of Zn in Hawassa soils, where added Zn was remained in the soil solution. Sorptions of Zn and Fe in the experimental soils followed Langmuir and Freundlich models, respectively. Zn adsorption was strongly related with soil organic carbon, calcium carbonate, iron and manganese contents of the soils, whereas iron was strongly related to clay. Fertilizer management for optimum crop production should therefore consider inclusion of Zn and Fe in the experimental sites, except Zn at Hawassa.

Keywords: Freundlich, Iron, Isotherms, Langmuir, Sorption, Zinc