Research HighlightsCan Soybean Hulls Become Another Nitrogen Source for Crops?
In this article, you’ll find details on:
- A North Dakota research project explored whether soybean hulls could be used as a supplemental slow-release nitrogen source for crops.
- After comparing applications of nitrogen sources, researchers found insignificant differences between them, and the N mainly wasn’t in the form that plants could use.
- Soybean hulls take a while to break down, indicating their usage may take more than one crop year to see results.
By Carol Brown
The soybean industry tries to use every part of the soybean so that nothing goes to waste. At harvest, the stems and pods go back to the soil to improve organic matter and replace nutrients. The beans are crushed for meal and oil. The hulls, a by-product of that crushing, are commonly used in livestock feed. But researchers continue to search for other uses of the hulls to improve their economic value.
Szilvia Yuja, a soils research specialist at North Dakota State University Carrington Research Extension Center, conducted a project to find out if a crop could use the nutrients contained in soybean hulls.
“In 2023, we applied soybean hulls shortly before planting spring wheat on research plots,” says Yuja. “I was hoping the wheat would use the hulls as a supplemental slow-release nitrogen source but we didn’t see significant differences in nitrate levels between the plots.”
Soybean hulls contain nutrients including nitrogen, potassium, and small amounts of phosphorus, sulfur and magnesium. They also have a fair amount of carbon (Figure 1). Yuja had soybean hulls analyzed at a lab for their nutrient content. This determined how much nitrogen she applied on the plots.
Yuja conducted the research on wheat plots with these comparisons:
- 40 pounds of nitrogen from soybean hulls
- 40 pounds of nitrogen applied as urea
- 90 pounds of nitrogen (optimum N rate) with a mix of 50 lbs. N from urea and 40 lbs. N from hulls
- 90 pounds nitrogen from urea only (optimum N rate)
- A check plot with no applied nitrogen.
The treatments were applied on dryland and irrigated plots, both with conventional tillage, as well as a dryland, no-till plot.
“We sampled soil from each plot after wheat harvest to measure the soil nitrates, but that didn’t really show the whole picture,” she explains. “We know that the nitrogen in the soybean hulls is initially tied up in organic forms, which are unavailable to plants. Eventually, through microbial activity, some or all of that nitrogen is expected to get released in plant-available forms such as nitrates or ammonium.”
Ammonium is a transient form of nitrogen and much of it is quickly converted to nitrates, she says. For this reason, they tested for nitrates when they sampled the soil to find out how much nitrogen was available to the plants.
“In this project, nitrate levels in the plots with the soybean hulls were no different than in the untreated plots,” Yuja comments. “This tells us that even though the nitrogen from the soybean hulls was added to the soil, it had not yet been converted to plant-available forms at the time of sampling, and therefore could not be taken up by the plants.”
She noted the soil nitrate levels were slightly higher on the irrigated plots, which was not surprising, she says, as moisture helps to break down the residues.
In research, a result that is not expected is still a result. This research project revealed that using soybean hulls as a supplemental nitrogen source didn’t produce a significant yield increase compared to where no nitrogen was applied in the year it was applied.
Published: Nov 11, 2024
The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.
