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