Adding Nitrogen to Soybeans — Can it Improve Yields?

    There has been a lot of talk in the industry about applying nitrogen on soybeans to drive yields to the next level. University researchers, industry agronomists and farmers have been trying it with mixed results. However, every so often, we learn that a farmer has used it successfully to increase yield.

    We all know soybeans are legumes that fix nitrogen. The question is do they fix enough nitrogen to produce 60, 80 or even 100 bushel soybeans?

    According to data from the International Plant Nutrition Institute (IPNI), soybeans require approximately 5 pounds of nitrogen to produce a bushel of grain. That adds up to 300, 400 or 500 pounds to reach 60, 80 or 100 bushel beans. That’s a lot of nitrogen and, obviously, more than the soil can provide naturally.

    IPNI also states that the maximum amount of nitrogen a soybean plant can fix is 300 pounds — equivalent to the needs of a 60 bushel crop.

    However, soybeans only fix about 50% to 60% of what it needs with rest coming from the soil, either as inorganic or organic mineralizable nitrogen.


    Kelly Cheesewright, a Dana, Ind., farmer has tried applying nitrogen on soybeans with mixed results. “To hit 70 and 80 bushel beans, we need to apply nitrogen. Soybeans can’t produce enough of its own nitrogen, and we need to figure out how to apply that extra nitrogen to get those high yields,” he said.

    Cheesewright has tried commercial fertilizers including urea and foliars applied in early August but has been disappointed. He has had better results with applications of turkey litter because its organic nitrogen releases more slowly and he gets a more consistent response. He also acknowledges that it takes the right weather to make everything come together.

    Jeff Littrell with HFR Farms in Stewartville, Minn., has been incorporating nitrogen into his soybean fertility program. “It comes down to what we saw at Kip Cullers’ farms in 2007 and that it takes nitrogen to build high yields,” said Littrell. “When we looked at all that he was doing, we knew we had to replicate it in our own way and build a program that works for us.”

    Littrell devised a complete fertility system where he applies in the spring at planting 100 pounds of dry 17-9-17 to the side of the furrow; 6 gallons of liquid that includes 2.5 gallons of ammonium thiosulfate, 2.5 gallons of calcium nitrate and 1 gallon of humates on the other side of the furrow plus a couple gallons of starter (8-16-3 plus phosphites) in-furrow.

    However, he considers his post-feeding program a very important part of this system. It includes side dressing 15 gallons of Cal-Zul (7 gallons of calcium nitrate, 7 gallons of ammonium thiosulfate and 1 gallon of humates) at V4 followed by weekly or biweekly foliar applications that include 2 pounds of N.

    “Our program doesn’t hinder but we feel actually enhances nitrogen fixation,” Littrell said. “Urea or 32% UAN (urea ammonium nitrate) will inhibit nitrogen fixation, but our use of calcium and humates stabilizes nitrogen and does not let it interfere with fixation.”

    “Everyone wants to put N on beans to increase yield and protein,” he told DTN. “But I believe we need to first look more closely at the overall fertility program and then fine-tune all its elements before just applying late-season nitrogen.”

    Ed Winkle, a crop consultant and farmer from Martinsville, Ohio, agrees that extra nitrogen is needed to produce high-yield beans. “It takes 5 to 6 pounds of nitrogen to produce a bushel of soybeans. On 100 bushel yield, that is 500 pounds of nitrogen,” said Winkle.

    Winkle thinks the first step is to focus on nitrogen fixation. “We always inoculate legumes in order to encourage them to produce all the free nitrogen from our nitrogen-laden atmosphere as possible. Big, healthy, pink-to-reddish nodules is my goal in every legume I raise. Inoculation always paid for me, but can it produce 5 to 6 pounds of nitrogen per bushel?” Optimizing the soil environment is also important, and he generally suggests applying half a ton of gypsum or lime to achieve proper soil ph.


    Many of the early nitrogen studies focused on applying N preplant. However, when there is excess soil nitrate or ammonium available, soybean plants get lazy and fix less nitrogen. That defeats the purpose of applying preplant N.

    For soybeans, N has to be top-dressed, side-dressed or applied as a foliar application. At the same time, it’s important to be cautious of leaf burn.

    The key is to maximize nitrogen fixation first, and then supplement with other forms of nitrogen from the soil or fertilizer. Before applying supplemental N, it’s also important to measure the N pool in the soil. There may be enough available to meet your yield goals.

    Applying N at the R3 stage has produced yield increases for irrigated soybeans in Kansas. In six out of eight study sites at Kansas State University, in-season applications of 20 to 40 pounds of N per acre resulted in yield increases of 5 to 10 bushels per acre. The six sites were low in organic matter and available soil nitrogen and grain yields exceeded 60 bushels per acre.

    N TIPS

    Growers interested in applying late-season N applications on soybean, consider the following tips compiled by Ron Gelderman,South Dakota State University:

    • Select fields with excellent yield potential and where soybeans have not been stressed and growth are excellent.
    • Select fields that have adequate soil moisture or are irrigated.
    • Analyze soils for organic matter, mineralizable N, and nitrate-N and use this information in deciding whether to apply N.
    • Apply N from full bloom (R2) through beginning pod (R3) or full pod (R4).
    • Apply 20 to 50 pounds of N either as liquid or dry urea either dribbled on the soil surface, or broadcast.
    • Apply a foliar feed with some N to make sure the plant’s metabolic machinery is working at optimal capacity. If treating the whole field, be sure to leave strips where no N was applied so you can evaluate the results.

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