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Busting the soybean fertility myth

An Illinois researcher says soybean crops don’t leave excess 
nitrogen in the soil for next year’s crop

I think the way we’re fertilizing soybeans is atrocious,” said Dr. Fred Belows, a professor and researcher at the University of Illinois. “Soybean does not add nitrogen to the soil. That’s another one of those urban legends.”

Addressing those comments to a group of farm journalists, Dr. Belows was a guest speaker at Case IH’s 2014 model-line launch in Denver, Colorado this past August. He presented a summary of his extensive research into crop production and fertility. Although all of his work has been done in Illinois, many of his findings are applicable to crop production, here, on the Canadian prairie.

His most surprising statements were on soybeans and the apparent misconception held by many producers that those crops don’t need nitrogen fertilization. Soybean, like alfalfa and other legumes, develops nodules on its roots that can fix atmospheric nitrogen and supply some of the plant’s fertilizer needs. But unlike the expectation with alfalfa, Belows said don’t expect soybean plants to produce an excess of nitrogen, let alone meet all their own needs.

Fertility management

“One of the reasons farmers don’t fertilize soybeans is because it can get some of its nitrogen from the environment, from the nodules,” he said. “We used to think for every bushel of soybean we got a pound of nitrogen in the soil. It’s the other way around. For every bushel of soybean you produce, you remove a pound of nitrogen from the soil.”

So, the reality, according to Belows, is farmers who want to maximize soybean yields will need to apply extra nitrogen. “Soybeans only get about 50 per cent of the nitrogen they need from the nodules,” he said. “The rest has to come from the soil. And soybeans is a crop that requires a large amount of nitrogen.”

But applying too much can delay or even prevent the formation of nitrogen-fixing nodules, and producers will loose the benefit of plant-produced, free fertilizer. “It’s one of those cases when you can’t have your cake and eat it too,” he continued. “Because if you put too much nitrogen on those plants, it will prevent nodule development or shut them right down if they’ve already developed.”

And Belows thinks overall fertility management on farms for all crop types may need to be rethought. “Soil testing was calibrated to yield in the ’60s,” he explained. “They’ve used the same recommendations for the last 50 years. In the ’60s, in the U.S., the average (corn) yield was 60 or 70 bushels per acre, 18,000 plants per acre. Now we’re growing double that yield with twice the plants.”

Rather than rely on soil test data, Belows believes developing a better understanding of plants’ fertilizer needs and developing application strategies to better suit them is what’s needed to boost future yields. That involves knowing when nutrients absorbed and where they are required in the plant. “You’re going to have to take advantage of technology to feed the plant what it needs when it needs it. I think you’re going to have to take a feed-the-plant approach.”

Six secrets to success

Below’s research is aiming to increase soybean yields in Illinois by 25 per cent by the year 2020. To achieve that, he has developed a trial program he calls “Six secrets of soybean success.” Each of those six secrets is a factor that boosts — or limits — crop yields.

First, is weather. But it’s something farmers can’t control.

Second, is soil fertility. Belows believes the fertility regime used by many farmers needs to be re-evaluated, ensuring plants have access to adequate nitrogen and phosphorus.

Third, is crop variety. Belows’ research has shown that yields can vary by up to 20 bushels per acre on the same fields simply because of variety selection. That is due mostly to varying disease resistance.

Fourth, is applying fungicides when necessary. Keeping photosynthetic activity going strong in the plant’s leaves during pod filling is essential to maximizing yields.

Fifth, is applying seed treatments. Ensuring early emergence and early season vigor is important. Young plants that suffer early stress cannot make up for that later in the season. “Apparently plants sense their fertility earlier than we realized and they make irrevocable growth decisions,” said Belows. “It’s all about rapid growth right from the start, because you can never make up for lost yield.”

Sixth, is row spacing. When planted at a reduced seeding rate, rows of about 15-inches allow for more space between plants within a row and increased branching. However, placing rows that closely can also promote more disease pressure due to reduced air circulation.

For more information on Dr. Below’s research visit the website †

About the author


Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.

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