Spreading growth

IN MEDIEVAL England peasants were permitted to graze their sheep on the lands of the nobility. There were no restrictions on how much their livestock could feed, but there was one ironclad rule: the peasants were not allowed to collect their animals’ droppings. Though the English nobles who came up with such regulations could not have known that the excrement was rich in nitrogen and vital for plant growth, they clearly knew that lands denied faeces were less productive. Today most farmers rely on synthetic fertilisers to do the nitrogen-enhancing job once reserved for dung. Urea, a compound of nitrogen, hydrogen, carbon and oxygen, can be made cheaply by mixing ammonia and carbon dioxide together at high pressure. The result is turned into pellets that can be scattered easily over fields.

Unfortunately, when such pellets are exposed to heavy rain, the urea they contain is quickly and wastefully washed away. A method of keeping it in place would thus be welcome. And Nilwala Kottegoda of the Sri Lanka Institute of Nanotechnology thinks she has one. As she and her team report in Nano, they have managed to bind urea molecules to a material that stops them dissolving too quickly in water. This material is hydroxyapatite, one of the components of bone.

Her choice of hydroxyapatite for investigation was no wild guess. It is already used to make capsules that release certain drugs slowly, in the way she wanted to achieve for urea. Hydroxyapatite is made by mixing phosphoric acid and calcium phosphate, so Dr Kottegoda simply added urea to the process. The result, she found, was that each molecule of the material clung on to six molecules of urea—a payload big enough to justify further testing.

To this end she and her colleagues steadily flushed water past samples of urea-enhanced hydroxyapatite held in tubes, while watching what happened using a spectroscope. The material shed its urea load gradually: 40% after 1,000 seconds; 60% after 2,000 seconds; 80% after 3,820 seconds. In contrast, when the researchers treated pure urea the same way, it was all gone in 320 seconds.

To find out whether the new fertiliser would make a difference in the field, Dr Kottegoda collaborated with some farmers near Sammanthurai, in eastern Sri Lanka. She ran tests on equal-sized rice paddies for four months. Some plots received no fertiliser at all. Some got pure urea equivalent to 100kg of nitrogen per hectare. Some got an amount of the newly created urea-hydroxyapatite that contained the same quantity of nitrogen as the pure urea. And in all cases the level of phosphorus (another important plant nutrient, levels of which were boosted incidentally by the hydroxyapatite) were adjusted to match from plot to plot.

The hydroxyapatite did, indeed, make a difference. Plots that received no nitrogen-based fertiliser at all averaged 5.5 tonnes of rice per hectare. Those that received urea alone yielded 7.25 tonnes per hectare. Those fertilised with urea-hydroxyapatite managed 7.8 tonnes per hectare.

Though the newly compounded fertiliser is more expensive to produce than its conventional equivalent, Dr Kottegoda calculates that this cost would quickly be offset if using urea-hydroxyapatite obviated the need to re-scatter fertiliser over a paddy after heavy rain—and that does not even take into account the increase in yield it brings with a single application. There might also (though she did not measure this) be a bonus reduction in the amount of phosphorus-based fertiliser a farmer needs to deploy in addition to nitrogen-based pellets. A simple idea, then. But a potentially important one.