Dissolution of Micronutrient-Doped Amorphous Calcium Phosphate Nanoparticles in Organic Acids: Tunability and Greenhouse Performance

Modern agriculture faces significant challenges in meeting global sustainability goals because conventional phosphorus (P) fertilizers dissolve prematurely and risk substantial runoff. Among slow-release fertilizers, amorphous calcium phosphate (ACP) offers a promising alternative to address both issues due to the slow and sustained P release, customizable properties, and not altering soil pH. Here, we investigated the dissolution properties of citrate-stabilized amorphous calcium phosphates (ACPc), both in pure form and doped with plant micronutrients (S, Fe, Zn, and Cu). Analysis of dissolution in 0.01 M citric and acetic acid buffers (pH 4.8) through dissolved ions and residual content analysis using ICP-OES, XRD, FTIR, and Raman spectroscopy methods showed that all materials maintained their amorphous structure, but ion-doping delayed the transformation of citrate-stabilized ACP into apatite, although the carbonate content decreased. Citric acid buffer enhanced the element release in undoped and Fe-doped ACPcs, while P release among doped materials remained almost unchanged. The acidity, complexation properties of acid buffers, and structural strain inherent within the material significantly influenced solubility, thereby affecting the dissolution behaviors. A greenhouse pot experiment performed to test the efficacy of doping ACPc on plant nutrition showed that corn (Zea mays) response was variable for micronutrient use efficiency: highest normalized uptake of Cu, high Zn shoot translocation, but poor Fe uptake to roots and shoots during the vegetative growth stage. Micronutrient doping affected both the material stability and dissolution kinetics, which are also reflected in the nutrient uptake by the corn plants. Overall, these outcomes provided useful insights for optimizing the delivery of P and micronutrient to agricultural plants.