The Role of Dual Organic Ligands in Tuning the Composition, Structure, and Properties of Fe-Based Metal–Organic Frameworks for Targeted Fertilizers

Organic ligands critically determine the composition and structure of metal-organic frameworks (MOFs). While iron-based MOFs have been studied as controlled-release fertilizers (CRFs), the influence of dual organic ligands on their structural and functional properties in agriculture remains poorly understood. In this study, three iron-based MOFs were synthesized: MOF1 (oxalic acid ligand), MOF2 (tartaric acid ligand), and MOF3 (1:1 dual-ligand mixture). Despite comparable nitrogen and iron contents, MOF1 and MOF2 formed uniform microcrystals, whereas MOF3 exhibited irregular, larger crystallites due to oxalic acid's dominant coordination, driven by its stronger coordination affinity relative to tartaric acid. Nutrient release studies revealed that MOF3 exhibited the fastest release kinetics, followed by MOF1 and MOF2, which were well-described by the Higuchi and Ritger-Peppas models. Soil simulation trials demonstrated that all MOFs significantly improved the fertility of saline-alkali soils, reducing pH (by 1.52 units) and salinity (by 6.75%), with MOF3 demonstrating the greatest efficacy. Pot and field trials further confirmed MOF3's superiority through promoting bok choy and maize growth in paddy soil, and increasing maize yield by 42.6% in saline-alkali soil. These results highlight the potential of dual-ligand MOFs as efficient CRFs and soil conditioners for sustainable agriculture.