For billions of people around the world, rice is a primary source of calories and protein. But growing the crop requires a lot of fertilizer , which can rapidly break down in the environment before plants are able to absorb the nutrients. Researchers have now developed a new time-released fertilizer that slowly discharges its cargo. When applied to rice fields in Sri Lanka, crop yields increased, even when only half the typical amount of nutrients was added.

It’s a “true win-win,” says Randy Jackson, an ecologist at the University of Wisconsin in Madison, who was not involved in the study.
 
A popular fertilizer is urea, a nitrogen-rich organic compound found in human urine. Urea is water soluble and volatile, which means that irrigation or a rain squall often sweeps it away in surface run-off or it escapes as a gas before it can be absorbed by plants. “Up to 70% of urea is lost to the environment,” says Nilwala Kottegoda, a materials scientist at the Sri Lanka Institute of Nanotechnology in Homagama and lead author of the study. That’s bad news for farmers—whose budgets are already stretched thin—and the environment: Fertilizer run-off into rivers, lakes, and deltas is a primary cause of algal blooms that are toxic to aquatic life.
 
Now, Kottegoda and her team have developed a new formulation of urea that works like a time-released drug capsule. The researchers attached urea molecules to hydroxyapatite—a constituent of human bones and teeth—in a six-to-one ratio by weight. The chemical bonds between the urea and hydroxyapatite molecules prevent the urea from decomposing too quickly. Yet, they do break down over time, which results in a controlled release of nitrogen at a rate that plants can absorb. Hydroxyapatite, which also slowly degrades, has the added benefit of being rich in phosphorus and calcium, elements that plants also need to thrive.
 
The researchers created roughly 90 kilograms of the fertilizer, which they shaped into 1-millimeter pellets. In the laboratory, Kottegoda and her colleagues ran water over pure urea and the urea-hydroxyapatite hybrid and compared how quickly the samples released nitrogen. They found that pure urea expelled 99% of its nitrogen within 5 minutes, unlike the hybrid, which took nearly a week.
 
The researchers then tested their fertilizer on a rice field in eastern Sri Lanka. They measured rice yields in three sections of a field: one that received no fertilizer, one fertilized with 100 kilograms of nitrogen per hectare of pure urea, and one fertilized with 50 kilograms of nitrogen per hectare from the urea-hydroxyapatite combo. The rows fertilized with the urea-hydroxyapatite duo yielded roughly 10% more rice than those fertilized with only pure urea, the team reports in ACS Nano.
 
This new fertilizer is made in a one-step process with inexpensive chemicals, unlike other controlled-release fertilizers on the market, such as polymer-coated urea.  It’s approximately 20% more expensive than pure urea, the researchers estimate. But it still may wind up saving farmers money, the authors contend, because it also delivers phosphorus and calcium, nutrients that normally have to be delivered by yet another set of fertilizers. “We believe that this novel formulation will be economically viable,” Kottegoda says.
 
Kottegoda and her colleagues are looking forward to testing their fertilizer on a perennial crop like tea. But no one yet knows how well the fertilizer works on other crops that aren’t constantly exposed to water. “The results might differ in wheat and maize,” Jackson cautions.
 
The author is Katherine Kornei,  a freelance science writer based in Portland, Oregon.