Layered Transition Metal Molybdates as New Slow-Release Micronutrient Fertilizer Matrices for Zn, Cu, and Mo

Abstract

Micronutrient availability in agricultural soils is an important driver for crop production, and for mobile micronutrients, slow-release fertilizers can more efficiently enhance crop nutrition. In this study, for the first time, layered transition metal molybdates (LTMs) are proposed as slow-release fertilizer compounds for the micronutrients molybdenum (Mo), zinc (Zn), and copper (Cu). A series of LTMs were successfully prepared in varying synthesis conditions, characterized (inductively coupled plasma optical emission spectrometry, X-ray diffraction, Fourier transform infrared, and scanning electron microscopy), and tested for solubility in water. For three selected LTMs (Zn− Mo, Cu−Mo, and Zn−Cu−Mo), the solubility as a function of solution pH was determined. The LTMs showed limited stability in acidic conditions. Therefore, co-compaction of LTMs with pH neutral or alkaline macronutrient carriers [muriate of potash (MOP), limestone, rock phosphate] was explored. A Zn-based LTM co-compacted in a MOP carrier showed a useful slow and almost constant release rate of both Mo and Zn in a column dissolution test (Mo release reaching 32% after 72 h), while a reference compacted MOP fertilizer with soluble Mo released 90% of Mo after 4 h. Hence, a Zn-LTM could prove useful as a slow-release fertilizer or raw material to incorporate into non-acidic macronutrient fertilizers.