Effect of mechanochemically modified MoO3–ZnO on Mo supply to plants when co-granulated with macronutrient fertilizers

Molybdenum (Mo) is an essential micronutrient required for plant growth but is prone to leaching from neutral and alkaline soils. The use of slow-release Mo sources could potentially reduce leaching losses from soils and increase crop yields. In this study, we assessed mechanochemistry as a green method to produce slow-release Mo sources. Molybdenum compounds (MoO3 or (NH4)6Mo7O24·4H2O) were mechanochemically (MC) treated with ZnO to synthesize compounds with a Mo content of 1–36%. Reduced Mo solubility after MC treatment, compared to the initial Mo source, was obtained with the MoO3 source and these composites were used for co-compaction with macronutrient fertilizers. Macronutrient pellets with 0.2% Mo were compacted using the 4% Mo and 36% Mo (characterized as ZnMoO4) compounds. A column dissolution test showed that the 4% Mo compound in a macronutrient carrier (DAP and MAP) only released around 40% of the total Mo compared to 80% for a non-MC treated control over 72 h. Column leaching using two soils revealed that the release behavior of Mo was strongly related to the pH of the leachate, which was affected by both the soil pH and the macronutrient carrier. More Mo was released when the MC-treated compound was co-compacted with diammonium phosphate (DAP) compared to monoammonium phosphate (MAP). The MC-treated compound with 4% Mo showed significantly less leaching than the control without ball milling when co-compacted with both MAP and DAP. In a pot trial with simulated leaching, the uptake of Mo was greater for the MC-treated 4% Mo compound co-compacted into DAP than for the other Mo sources. Overall, our results indicate that MC-treated MoO3–ZnO could be used as a slow-release Mo source in high-rainfall areas.