Phosphorus Diffusion from Fertilizer: Visualization, Chemical Measurements, and Modeling

Phosphorus (P) applied to soil as fertilizer moves away from the point of application mainly through diffusion. Sorption and precipitation reactions may reduce its mobility and availability to plants. Here, we developed a method to visualize diffusion of P from fertilizer in a simple and nondestructive way. A fertilizer granule is added in the center of a soil‐filled Petri dish. After a given incubation time, a filter paper impregnated with Fe oxide acting as a P sink is placed on the soil surface. The P captured on the filter paper is visualized using a modified malachite green method, which creates a mirror image of the diffusion zone. The paper is scanned and analyzed using imaging software to quantify the extent and intensity of the diffusion zone. This method was applied to a range of fertilizer formulations in soils with contrasting properties. The extent of diffusion was also assessed through concentric sampling and chemical analysis of soil sections. The measured diffusion was compared with predictions of a numerical diffusion model. The method demonstrated the effects of soil properties and fertilizer formulation on P diffusion. The extent of diffusion strongly depended on soil properties, with least diffusion in soils rich in amorphous oxides and in calcareous soils. Results of the visualization method were in good agreement with chemical analyses and modeling results. This method allows easy comparison of fertilizer sources and enables a better understanding of the physicochemical processes affecting fertilizer P behavior at the soil‐fertilizer interface.