Synthesis of Nano-Sized Metavivianite and Nano-Sized Chitosan-Coated Metavivianite and Their Solubility in a Calcareous Soil Saturated Paste Extract with Soil Microbe Inoculation

Iron (Fe) deficiency plays a critical role in plants grown on calcareous soils due to high pH and carbonate levels that limit Fe availability. Synthetic Fe-chelates are commonly used to address this issue, but they pose risks of leaching and groundwater contamination. Nano-sized fertilizers like nanometavivianite (nano-metaVT) and chitosan-coated nano-metaVT (nano-CT-metaVT) offer a promising alternative by targeting the rhizosphere more effectively. Despite previous success with bulk metaVT in mitigating Fe deficiency, its extremely low solubility (Ksp = 10–36) in dilute solutions has raised questions about its efficacy. This study investigates the solubility of nano-metaVT and nano-CT-metaVT in diluted electrolyte solutions (KCl) and calcareous soil-saturated paste extract (SPE), which contains soil-relevant organic compounds like low-molecular-weight organic acids, amino acids, fulvic acid, and humic acid. Additionally, the impact of beneficial soil microbes─Pseudomonas chlororaphis O6 and Pseudomonas protegens Pf-5─on solubility was examined. Findings reveal that the chitosan coating enhances the solubility of nano-CT-metaVT in SPE suspensions. Inoculation with Pp Pf-5 significantly increases the total soluble Fe concentration for both nano-metaVT and nano-CT-metaVT, not only by lowering environmental pH but potentially through other unidentified mechanisms. The results highlight key factors affecting the solubility of nano-metaVT fertilizers, such as coating materials, soil microbe, and organic ligands in the soil environment. The study provides insights into the potential application of nano-metaVT and nano-CT-metaVT by analyzing the characterization, solubility, and impact of soil microbe inoculation on both nano-metaVT and nano-CT-metaVT in SPE suspension.