Nanofertiliser works with less pollution

RESEARCHERS in Sri Lanka and the UK have developed a slow-release nanofertiliser, which gradually releases nutrients over the space of a week and could boost crop yields.

Slow-release fertilisers for urea, an essential nitrogen-containing compound, already exist, and consist of urea coated in water-soluble sulphur or polymers. These fertilisers, however, are expensive and therefore unsuitable for use in developing countries. It is thought that the high cost of fertiliser may be a major limiting factor for food supplies in such countries. Conventional urea fertilisers tend to release much more urea than is necessary, which converts into ammonia before the plants can absorb it, leading to nitrogenous run-off which pollutes waterways and causes algal blooms, known as eutrophication. The researchers, led by Gehan Amaratunga from the University of Cambridge, UK, decided to investigate whether the kind of slow-release nanotechnology found increasingly in pharmaceuticals might hold the answer.

Gehan and the team decided to use hydroxyapatite nanoparticles. They are a useful source of phosphate, another important plant nutrient, have excellent biocompatibility and the nanoparticles have a high surface area. The final particles have a ratio of 6:1 of urea to hydroxyapatite. They are synthesised in a one-step process. A phosphoric acid solution is added dropwise to a solution of calcium hydroxide and urea. The mixture is then flash-dried, giving a dry, nanoparticle fertiliser. The researchers scaled up the process to make 87.5 kg in one batch.

In a rapid water-release test, the nanofertiliser released nitrogen 12 times slower than conventional urea fertilisers over the course of a week. The researchers also tested the nanofertiliser in field trials on rice crops in Sri Lanka. The nanofertiliser released just half the amount of urea as conventional fertilisers, but rice yields were boosted by 10%. Fields using conventional urea fertiliser at recommended rates had yields of 7.3 t/ha. Those using the nanofertiliser, containing 50% less urea, had yields of 7.9 t/ha.

“This work demonstrates that nanotechnology can be used to develop slow-release fertilisers which can significantly reduce the amount of chemicals used while maintaining yield,” the researchers say.

ACS Nano doi.org/b2d9