Zinc in ammoniated phosphate fertilizers: Solid‐phase speciation, solubility, and use of barrier coatings to enhance zinc availability

Zinc (Zn) is the micronutrient commonly found deficient in agricultural soils worldwide, limiting crop yield and reducing food quality. Zinc‐enriched fertilizers have been successfully used to tackle crop Zn deficiency. However, Zn solubility is reduced after addition to phosphate fertilizers due to the formation of sparingly soluble precipitates, decreasing Zn availability to crops. We hypothesized that the availability of Zn in the fertilizer to plants is related to its speciation in the fertilizer. We evaluated a range of ammonium phosphate fertilizers using X‐ray absorption near‐edge structure and assessed the water‐solubility of Zn in the formulations. Four Zn species were identified in these fertilizers, with zinc ammonium phosphate the most abundant one. The speciation of Zn in the fertilizer had little relationship with the water solubility of Zn in the final product. Zinc solubility was driven by fertilizer pH, with lower pH resulting in higher solubility. We concluded that added Zn reacts with the fertilizer matrix to form mainly zinc (ammonium) phosphates, and when the fertilizer is dissolved in water, hopeite controls the solubility. Based on these findings, we tested whether a barrier coating between the P granule and the Zn compound could prevent reaction between Zn and P and thus enhance Zn availability. Indeed, higher Zn uptake was observed in an isotopically labeled fertilizer growth trial when a barrier coating physically separated Zn and P in the fertilizer. In summary, Zn availability to crops can be maximized by decreasing fertilizer pH and separating P and Zn in the granule.