Adsorption_Desorption Characteristic of Zinc, Copper, Manganese and Iron in Calcareous Soils and its Correlation with Physical and Chemical Properties of Soils and Micronutrient Uptake by Wheat

Micronutrients in calcareous soil are severely sorbed and thus may being made unavailable for plants. Therefore, investigating the sorption and desorption characteristics of micronutrients in these soils is more important. These findings help us to decide how micronutrients must be applied. In this project, twenty soil samples from wheat fields were brought into equilibrium with solutions containing 5-2000 mgL-1 Zn and/or Cu at 25°C for studying the adsorption behavior of these micronutrients. Data were fitted with adsorption equations and then equation coefficients were correlated with soil characteristics. Desorption of Zn were determined by using CaCl2 and DTPA. In pot experiment, wheat was cultivated in all soils treated with there levels of Zn and Cu (0, 5 and 10 mg kg-1). After eight weeks, plants were harvested and shoot dry weight, Zn and Cu concentration and uptake was determined. Correlations between plant response and coefficients of adsorption equations were investigated. Results showed that adsorption behavior of Zn and Cu could estimate well with Langmuir, Freundlich and Temkin equations. Zn and Cu adsorption best fitted with Freundlich and Langmuir, respectively. Two-surface adsorption properties were observed for Cu. Cu sorbed more than Zn in experimental soils. Zn adsorption maxima (coefficient of b in Langmuir equation) was between 10024 and 54461 µg g-1. Cu adsorption maxima ranged in first surface (50-500 mg L-1 Cu in equilibrium solution) between 7246 and 12354 µg g-1 and in second surface (500-2000 mg L-1 Cu in equilibrium solution) varied from 33908 to 319493 µg g-1. Cu adsorption energy (coefficient of k in Lanmuir equation) in first surface was markedly more than in second surface. Organic carbon had the best correlation with Zn adsorption. Regression equations showed that Zn adsorption maxima increased with increasing in calcium carbonate content in soils. In equilibrium solutions with low Zn content (less than 100 mg L-1) DTPA desorbed Zn with more strength than CaCl2 did, however, in higher Zn containing equilibrium solutions CaCl2 desorbed more Zn. Desorption of Zn by CaCl2 and DTPA was averagely less than 10 percent of Zn adsorbed. Among soil characteristics, Cu adsorption significantly and negatively related to organic carbon and available Cu (extracted by DTPA). Application of Zn and Cu increased shoot dry weight and Zn and Cu concentration and uptake by wheat. Plant response was affected differentially to Zn and Cu application in deferent soils. Shoot dry weight in 50 percent of soils increased with Zn and Cu application especially at 10 mg kg-1. Correlation between plant response and Zn and Cu adsorption was not significant