Effect of aging on the availability of zinc added to a calcareous clay soil

The effect of aging on the availability of Zn added to a slightly calcareous soil was studied using corn in a pot trial after incubation of the soil with Zn as ZnSO₄ for 0, 1 and 2 years. Meanwhile, the effect of aging on the forms and extractability of Zn added to the soil were studied by a sequential extraction procedure and three successive extractions with 0.005 mol/l DTPA + 0.1 mol/l TEA (triethanolamine)+0.01 mol/l CaCl₂ at pH 7.3. The incubation and glasshouse study provided direct biological evidence that long-term aging decreases the availability of Zn added to soil. When water-soluble Zn was added to a soil, it changed gradually into less available forms, mostly into the forms associated with Fe (Al) oxides and an unreactive residue (minerals), such that Zn uptake by corn decreased with incubation time. The processes (aging) associated with this decrease in availability of Zn added to soil could be described by a diffusion equation. The aging rate is inversely proportional to the square root of time. It appears that the processes which lead to the decrease in availability of Zn added to soil are controlled by the process of micropore diffusion.