The standard free energy change for adsorption process was negative suggesting that the specific adsorption of zinc-chelates by these soils was spontaneous. The standard free energy change of adsorption in calcareous soil varied in the order: ZnSO4 > Zn-FA > Zn-EDTA = Zn-DTPA. The enthalpy change of Zn adsorption (ΔH°) showed negative value in all the soils indicating that adsorption of Zn was an exothermic reaction. The higher values of enthalpy change of Zn adsorptionfrom ZnSO4 in soils than zinc chelates suggest a greater incorporation of zinc in crystal lauice of clay as compared with zinc-chelates. Adsorption of zinc-chelates in calcareous soils conformed to the Langmuir adsorption isotherm. Stepdown multiple regression analysis revealed thatfree CaCO3 and organic matter are important inpredicting adsorption of Zn in soil. Adsorption of zinc in calcareous soil increased with increasing temperature indicating chemisorption of zinc in soil. The supply parameter of Zn through Zn-chelates to plant roots was greater than that of ZnSO4 in majority of soils. The supply parameter of Zn increased with increasing levels of applied zinc in soil. The highest supply parameter was observed at 50°C.
Zn-chelates, adsorption, change of free energy, entropy, enthalpy, supply parameter
