Potassium-Doped Amorphous Calcium Phosphate Nanoparticles Functionalized with Urea as a Sustainable Controlled-Release NPK Fertilizer.

Conventional fertilizers are associated with substantial nutrient losses and environmental impacts, highlighting the need for more sustainable fertilization strategies. Here, we report the synthesis of potassium-doped amorphous calcium phosphate nanoparticles functionalized with urea (KACP-U) as a controlled-release nitrogen-phosphorus-potassium (NPK) nanofertilizer. Structural and compositional analyses confirmed the amorphous nature of the calcium phosphate matrix, effective potassium incorporation, and successful urea loading. Soil column leaching experiments under severe simulated rainfall conditions demonstrated that KACP-U significantly mitigates nutrient losses, with sustained urea release and a marked reduction of phosphorus, calcium, and potassium leaching by more than 1 order of magnitude compared with conventional fertilizers. Greenhouse experiments with corn (Zea mays) provided evidence that the controlled-release behavior of KACP-U observed in the leaching experiments was maintained under greenhouse conditions. KACP-U reduced phosphorus and calcium leaching by approximately 35% and urea (an N-containing compound) leaching by approximately 75%, with respect to a physical mixture of monocalcium phosphate, potassium chloride, and urea. At the same time, shoot biomass remained comparable to the control, while root biomass significantly increased by more than 47%. Notably, soil pH remained close to that of untreated soil, indicating a potentially lower risk of soil acidification compared with conventional nitrogen fertilizer. Results also indicate increased Ca and Mg accumulation in roots, by approximately 35 and 90%, respectively, compared with conventional fertilizer. This study underscores the potential of amorphous calcium phosphate-based nanofertilizers as a sustainable alternative to conventional NPK formulations.