Diffusion and solubility control of fertilizer-applied zinc: chemical assessment and visualization

Abstract

Background and aims Sorption and precipitation reactions reduce the mobility of fertilizer-applied zinc (Zn), and hence reduce availability to plants. Here, we developed a method to visualize diffusion of Zn from fertilizers. Methods Zinc diffusion was assessed for three fertilizers in three soils of varying pH. The fertilizer (ZnSO₄-coated monoammonium phosphate, MAP; ZnSO₄-coated diammonium phosphate, DAP; or ZnSO₄ alone) was applied in the centre of a Petri dish filled with soil. The Zn diffusion zone was visualized at various incubation times (1, 7 and 18 days) by exposing a CaCO₃-impregnated filter paper on the soil surface and colouring the Zn captured on the filter paper using dithizone. The extent of diffusion was also assessed through concentric sampling and chemical analysis of soil sections. Results The visualization results agreed with the chemical analyses and demonstrated the effects of soil properties and fertilizer formulation on Zn diffusion. Combining Zn with P fertilizer restricted its diffusion, particularly with DAP fertilizer or in high pH soils. Solubility considerations indicated that this was due to the formation of Zn phosphate minerals. Conclusions The visualization method is a useful tool for studying diffusion of Zn from spot applications and allows easy comparison in Zn diffusion between fertilizer sources.