Layered Double Hydroxides as Slow-Release Fertilizer Compounds for the Micronutrient Molybdenum

Molybdenum (Mo) is an essential plant micronutrient. Despite low plant Mo requirements, deficiencies are not uncommon and soluble Mo fertilizers are often applied. However, soluble Mo may result in poor Mo use efficiency due to strong sorption (acid weathered soils) or leaching (lighter-textured soils). Here, ZnAl layered double hydroxides (LDHs), loaded with molybdate (MoO₄), were examined for their potential as slow-release Mo compounds. Chloride-exchanged LDHs with varying Zn/Al ratios (2, 3, and 4) were exchanged with MoO₄. Zn₂Al LDH indicated MoO₄ intercalation, whereas Zn₃Al and Zn₄Al LDHs bound MoO₄ merely on edge sites. Short-term Mo-LDH incubation identified sulfate, carbonate, and phosphate as the most competitive anions for MoO₄ exchange. Long-term Mo-LDH incubation in simulated pH-neutral soil solutions demonstrated slow Mo release from Zn₂Al LDH (half-life of 35 h), with a total Mo desorption of up to 85%. For Zn₃Al and Zn₄Al LDHs, Mo desorption was limited to <20%. Finally, several macronutrient fertilizers were tested as possible carriers for Mo-LDH fertilizer compounds.