The effect of orthophosphate‚ nitrate, and chloride anions on desorption and availability of adsorbed zinc in some calcareous soils

Nowadays, the use of zinc (Zn) fertilizers has been expanded to satisfy the deficiency of this element and improve the yield and quality of agricultural products. The knowledge about the availability and release of adsorbed Zn after application in soil is necessary to achieve the best fertilization management and soil and water conservation against Zn accumulation in soil. On the other hand, presence of anions in irrigation water, agricultural fertilizers, and sewage sludge can affect adsorption, desorption, and availability of nutrients such as Zn. Zinc adsorption characteristics was usually studied using isotherm coefficients; while availability of adsorbed Zn in soil is important in soil fertility. In this study, the effects of orthophosphate, nitrate, and chloride anions on adsorption and desorption capacity were investigated in five calcareous soil of Chaharmahal - Va - Bakhtiari province.

In this study, a solution containing concentrations of 25, 50, 75, 100, 150, and 200 mg l-1 of Zn as ZnSO4 source in the presence of KH2PO4, KNO3 and KCl electrolytes (50 mM) was used. After Zn adsorption in soils, availability and desorbed of Zn was measured by DTPA-TEA and 0.01 M CaCl2, respectively. The amount of Zn desorbed in 0.01M CaCl2 is adsorbed Zn as non-specific. The adsorbed Zn as specific was calculated from the difference between the amounts of adsorbed Zn and desorbed Zn by 0.01 M CaCl2.

According to the results, the highest amount of Zn adsorbed as specific in the presence of all anions. Percentage of adsorbed Zn in all soils as specific ranged from 99.65 to 99.80 in the chloride solution (more than other anions p<0.05), 99.84 to 99.99 in the nitrate solution, and 99.55 to 99.72 in the orthophosphate solution. Availability of adsorbed Zn ranged from 41 to 43% in orthophosphate solution, 49 to 54% in nitrate solution, and 58 to 61% in chloride solution.

The results showed that the amount of adsorbed Zn as specific was more than amount of adsorbed Zn as non-specific in the presence of all the anions in all studied soils. The result showed that highest amount of available adsorbed Zn was extracted in the presence of chloride, nitrate, and orthophosphate (P<0.05). About 50% of the Zn adsorbed extracted by DTPA-TEA. In the presence of all studied anions, more than 99% Zn adsorbed as specific. Therefore, Zn adsorbed at specific sites and 0.01 M CaCl2 cannot extracted it. The results of this study showed the application of P and Zn as fertilizers in calcareous soils can lead to a reduction in extracted Zn by DTPA-TEA in treated soils with these nutrients.