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Encapsulated fertilizer improves nutrient release, protects water quality

conventional, left, and encapsulated fertilizers
Unlike conventional fertilizer, on the left, the encapsulated biochar-fertilizer combination, on the right, is designed to control nutrient release, making them available when the plant needs them. The encapsulated fertilizer will help reduce production costs and protect water quality.

The secret is in the timing.

Chemical fertilizers should activate precisely when the crop needs more nutrients.

To accomplish this, South Dakota State University researchers are developing an encapsulated biochar-fertilizer combination designed to control nutrient release, making them available when the plant needs them.

“We’re taking precision ag to the next level,” said Assistant Professor Lin Wei of the Department of Agricultural and Biosystems Engineering. He began developing what he refers to as a smart fertilizer two years ago.

Crop production relies heavily on fertilizers, which account for 30 percent of production costs. However, an estimated 30 to 50 percent of those nutrients run off into lakes or streams or leach into groundwater, Wei explained. “We are increasing fertilizer efficiency while improving the soil’s ability to retain moisture and nutrients. This will help reduce production costs and protect water quality.”

Precision agriculture technologies are helping producers vary fertilizer and water application rates across their fields, but Wei said, “It’s not just about putting the right fertilizer on the right spot, but synchronizing nutrient release with plant uptake to minimize nutrient losses—that’s even more precise.”

To do this, Wei collaborates with Associate Professor Yajun Wu of the Department of Biology and Microbiology in the College of Natural Sciences. One doctoral student and one master’s student are also working on the project.

The research is supported by the North Central Regional Sun Grant Center and U.S. Department of Agriculture Hatch Act funding through the South Dakota Agricultural Experiment Station.

Lin Wei and Zhisheng Cen discuss column testing results
Assistant professor Lin Wei and graduate student Zhisheng Cen discuss results from column testing.

Formulating fertilizer

First, the researchers combined biochar with nitrogen fertilizer to form 1/8th– to 3/16th-of-an-inch pellets. “Soil microbes thrive in biochar pores and surfaces, which helps increase organic matter and microbial activity,” Wei said. That then helps improve water retention, thus minimizing nutrient runoff and leaching.

Next, the research team coated the pellets with a very thin layer of biodegradable polymer to control compound release. When the plant roots touch the smart fertilizer, the nutrients begin to release.

“The root extrudes a chemical that interacts at a molecular level to open the coating,” Wei said. This causes the nutrients to become available to the roots as they develop, thus  increasing use efficiency and reducing fertilizer run-off and leaching to improve water quality.

In addition, the degradable coating protects the environment, he noted. “There will be no polymer residue left behind.”

Testing runoff, root growth

Column testing using sandy soil showed the smart fertilizer significantly reduced leaching and runoff compared to conventional fertilizer. Only 10 to 20 percent of the nitrogen from the smart fertilizer washed through the column, while 50 to 60 percent of the nitrogen from the conventional fertilizer did so.

However, Wei hopes to improve those numbers. “We think we can improve the coating layer, thereby reducing leaching to a single-digit percentage,” he said. “That’s our target.”

In addition, the researchers used corn as a model crop to examine the smart fertilizer’s effect on root growth. Preliminary testing in the greenhouse showed a significant difference between plants grown with and without the smart fertilizer. “The root system is much larger because the fertilizer slowly releases the nutrition. Results were encouraging,” Wei said. The researchers are applying for external funding to do further testing in the lab and in the field.

Though different types of crops require additional nutrients at different times, the mechanisms that trigger nutrient release are the same, Wei explained. “This project is about increasing the sustainability of agricultural production—we’re giving precision agriculture one more tool.”