Role of Cation Substitution and Synthesis Condition in a Calcium Phosphate-Based Novel Nanofertilizer on Lettuce (Lactuca sativa) Yield

Hydroxyapatite nanoparticles (HANPs) have received increasing attention as a potential novel fertilizer. Although stoichiometric HANPs have limited solubility, they can be tuned for more controlled phosphorus (P) release applications. To enhance the dissolution for maximizing P delivery to plants, we synthesized HANPs and dicalcium phosphate anhydride (DCPA) under varying crystallization conditions (temperature, pH, and cation substitution). The efficacy of these materials in improving lettuce (Lactuca sativa) yield was investigated in the greenhouse. Comparative analyses of shoot and root biomass, tissue nutrient content, and P loss in the leachate show that all HANPs and DCPA stimulated lettuce growth, but the extent of enhancement was a function of synthesis condition. Lettuce fertilized with DCPA, a more soluble Ca-P product, showed two times higher crop yield than controls, but P loss in leachate was the highest. On the other hand, lettuce fertilized with HANPs synthesized at pH 7.0 resulted in a 73% greater crop yield than the control, and the P loss to leachate was lowest among all Ca-P products tested. Considering the 'green' efficiency that accounts for both promoting plant growth and limiting P loss, HANP synthesized at pH 7.0 is found to be more optimal. These results demonstrate that tuning the properties of HANPs is an effective approach to optimize P release to enhance crop yield and minimize P loss. These outcomes also provide a scientific basis for developing tunable nanofertilizers specific to a crop or soil condition and, thus, a sustainable approach toward meeting global food demands while simultaneously minimizing the negative environmental impacts of agriculture.