To develop a method to visualize diffusion of zinc from fertilizers.

• Zinc diffusion was assessed for three fertilizers in three soils of varying pH.
• The fertilizer (zincSO4-coated monoammonium phosphate, MAP; zincSO4-coated diammonium phosphate, DAP; or zincSO4 alone) was applied in the centre of a Petri dish filled with soil.
• The zinc 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 zinc captured on the filter paper using dithizone.
• The extent of diffusion was also assessed through concentric sampling and chemical analysis of soil sections.
• The visualization results agreed with the chemical analyses and demonstrated the effects of soil properties and fertilizer formulation on zinc diffusion.
• Combining zinc 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 zinc phosphate minerals.