Tailoring hydroxyapatite nanoparticles to increase their efficiency as phosphorus fertilisers in soils

Hydroxyapatite nanoparticles (HA-NPs) have been proposed as a novel phosphorus (P) fertiliser that could potentially provide increased efficiency through controlled release. In the present study, three types of HA-NPs were synthesised with differences in surface charge and hydrodynamic sizes before being incubated in two P deficient soils (an Ultisol and a Vertisol) in the laboratory for up to 240 d. The three types of HA-NPs had surface charge values of + 21 (+/- 4.3), 0 (+/- 3.5) and -12 (+/- 2.7) mV in a neutral medium (pH 7). In the Ultisol (pH 4.7), the addition of HA-NPs resulted in a smaller initial increase in P availability than triple superphosphate (TSP) due to their lower solubility. However, P availability in the TSP treatment decreased over time due to adsorption of P to the soil constituents, with a concomitant increase in the NaOH-extractable P fraction. In contrast, the release of P from the HA-NPs remained relatively constant over the entire incubation period. After 45 d, the soils amended with negatively charged HA-NPs had higher levels of available P when estimated using diffusive gradients in thin films (DGT-P) compared to both the neutral and the positively charged HA-NPs. In the Vertisol (pH 8.2), although the addition of TSP increased P availability markedly, amendment with the three HA-NPs did not increase P availability due to the low solubility of hydroxyapatite at this high pH. Indeed, for the Vertisol, most of the P added as HA-NPs remained in the Ca-P fraction. Our results show that for the acidic Ultisol, HA-NPs with altered surface charges are potentially a useful form of slow release P.