Phosphate-Exchanged Mg-Al Layered Double Hydroxides: A New Slow Release Phosphate Fertilizer

The global phosphorus crisis provided impetus to develop fertilizers with better P use efficiency. We tested layered double hydroxides (LDHs) as slow release fertilizers with superior performance to fertilize strongly P-fixing soils. Mg-Al LDHs with varying M2+/M3+ ratios were synthesized as NO3- forms and were exchanged with HPO42-. XRD and XANES spectroscopy confirmed the identity of the phosphate-exchanged LDH. Decreasing the M2+/M3+ ratio, i.e., increasing the anion exchange capacity, increased the selectivity of P adsorption due to the increasing charge density of the LDH layers. The fertilization efficiency of the phosphate-exchanged LDH (Mg/Al ratio of 2) was compared to that of a soluble P fertilizer in two P-deficient soils, an acid weathered soil and a calcareous soil. The P use efficiency of the P-LDH in the acid soil was up to 4.5 times higher than that of soluble P. This was likely related to a liming effect of the LDH. In the calcareous soil, the P use efficiency at low doses was only 20% above that of soluble P, whereas it was lower at high doses. These overall encouraging results warrant further studies on the boundary conditions under which P-LDHs may outperform traditional fertilizers.