The use of hydroxyapatite nanoparticles as novel phosphorus fertilisers to enhance phosphorus mobility in tropical soils

Phosphorus (P) deficiency causes yield losses in tropical regions due to the strong P sorption of tropical soils attributed to the high amount of iron and aluminium oxyhydroxides and the low pH. Novel P fertilisers such as hydroxyapatite nanoparticles (HAP-NPs) have been proposed to improve the P application efficiency. Such nanoparticles (< 100 nm) potentially have higher mobility and bioavailability in strongly sorbing soils as they interact less with sorption sites and have a more controlled P release than soluble P fertilisers.

In a diffusion experiment using tropical soil (pH 5), the P mobility of liquid and powdered HAP-NPs was investigated and compared with that of liquid K₂HPO₄ and powdered TSP, all added at an equal P dose located at the centre of a soil column. A novel sampling method using Diffusive Gradients in Thin Films (DGT) was applied that could distinguish colloidal P from dissolved P. The soil was sampled with a back-to-back pairwise system of a classic and a nano DGT. The nano DGT consists of a normal DGT but with the addition of a dialysis membrane between the diffusive membrane and the binding layer to exclude all colloidal particles. Therefore, the difference in DGT-P between both DGTs could be attributed to P-loaded colloids or nanoparticles.  In addition, standard soil extractions (1 mM CaCl₂ and oxalate extractions) were performed to compare with the DGT method.

After one week of incubation, colour visualisation of the gels showed that the HAP-NPs were still mobile and colloidal when applied as a liquid. In contrast, the liquid K₂HPO₄ fertiliser no longer yielded mobile P. Both powdered HAP-NPs and TSP did not yield any mobile P after one week.  Results after one month are pending, but the HAP-NPs are expected to produce more bioavailable P than K₂HPO₄ and TSP, as the HAP-NPs dissolve slower and adsorb less strongly to the soil's binding sites. These results will help to understand the fate of  HAP-NPs in a tropical soil and how they may improve P application efficiency.