Application of Layered Double Hydroxides as a Slow-Release Phosphate Source: A Comparison of Hydroponic and Soil Systems

The utilization of slow-release fertilizer materials capable of responding to their environment and releasing nutrient ions efficiently over a prolonged period is an emerging research area in agricultural materials sciences. In this study, two-dimensional layered materials were prepared to release phosphor ions (P) slowly into the soil as well as in the hydroponic system. Various P-intercalated layered double hydroxides (LDHs) (Mg/Al, Zn/Al, and Mg-Zn/Al-LDHs) with a molar ratio of 2:1 were synthesized using an ion-exchange method from corresponding LDHs containing NO₃^- ions within the layers. Sodium alginate (SA) was used to encapsulate P-intercalated Mg/Al-LDH to produce bionanocomposite beads (LB) to check the effect of the biopolymer matrix on the release characteristics. The prepared materials were characterized by XRD and FTIR to confirm the incorporation of P in LDHs. TGA, SEM, and elemental analysis were also performed to study the thermal decomposition pattern, surface morphology, and chemical composition of synthesized materials. The P-release experiments were conducted in a soil solution. The performance of the prepared materials was investigated in soil as well as in a hydroponic system for tomato plants under a controlled atmosphere of humidity, temperature, and light. The fertilization ability of the prepared materials was compared with that of a soluble P source (KH₂PO₄), commercial hydroponic fertilizer (Nutrifeed), and a commercial soil slow-release fertilizer (Wonder plant starter). The prepared materials demonstrated a slow release of P in the soil solution. P-intercalated LDHs were not very effective under hydroponic conditions; however, the LDHs were more effective in the soil system in terms of dry matter production and P content in dry matter. Furthermore, LDHs were able to increase the soil pH value over time.