N on Soybeans
Moving from radical to reasonable
By: Dr. Jim Ladlie and John Oolman

Out in the soybean fields, beneath the soil on plant roots, billions of microbial "factories" are hard at work. These factories are nodules of bacteria that take nitrogen (N) from the air and reduce it to ammonium. These bacteria, called Bradyrhizobia, fix nitrogen into a form useable by the soybean plant. In return, the plant provides the bacteria with food in the form of carbohydrates.    Because of this symbiotic relationship, agronomists have generally believed that the soybean plant could fix enough nitrogen for high yields and thus advised against N fertilization.    However, in high yield conditions, fixation provides only about half of the soybean plant's total N needs.
    At the R5.5 stage the plant focuses on filling the seeds in the pod and moves all of its nodule N to the seeds. Thus, nodulation stops at R5.5, when the plant stops sending carbohydrates to the roots. From this point on, the plant still has 40 to 50% of its seasonal N to take up.
    Looking at the soybean plant's seasonal nitrogen needs-275 lbs/A to produce 50 bu/A-and its use rate, many scientists have studied applying N fertilizer to soybeans. This theory makes sense. If N fixation provides only half of the plant's N needs, then the rest must come from the soil or from fertilizer as nitrate or ammonium. However, many researchers found no yield increase from supplemental N or their results were inconclusive (see sidebar).
    Yet, N on soybeans is paying for some farmers and researchers.
    Dr. Jim Ladlie, president of Agri-Growth, has traveled throughout the United States and Canada since 1979, training farmers and other ag professionals on crop production. In north central Nebraska, he met farmers who were raising 65 to 85 bu/A soybeans. (The national average in 1996 was 37.6 bu/A.) In this area of Nebraska, the soils are sandy and farmers irrigate their crops. To achieve these high yields, farmers use starter and fertigation. Also, for every inch of irrigation water, they recycle 5 to 8 lbs/A of nitrates from the ground water.
Ladlie decided to study N on beans further after:
• Studying the fact that soybeans need 5.5 lbs/N per bushel of grain and stover
• Learning that soybeans take up 40 to 50% of their N after seed fill begins
• Meeting farmers who increased soybean yields with supplemental N.
   Agri-Growth researched N on beans across fields in 1996 and 1997. The results showed a yield increase from applying N.
   Timing appears to be the key. Applying N at the right time can increase yield through higher numbers and weight of seeds. Soybean plants differ from corn plants. Corn plants can handle all their N up front because they have no nodulation and take up N as needed. Soybeans, which have nodulation, need to be spoon fed N.
    "To obtain higher yields, you cannot treat a soybean plant like a corn plant. It can't handle it," Ladlie said. "You have to treat the soybean plant like a stand-alone plant, not a stepchild to corn or cotton." He recommends farmers consider the following:
• Applying N, up to 30 to 50 lbs/A, before or at planting can satisfy the soybean plant's early N needs. This is especially true with:
Early planting
Drought conditions
High residue situations
Eroded or compacted soil
Low soil N or organic matter levels
Soil pH levels below 6.0 or above 7.0
Early N applications can, at times, actually stimulate nodulation because N helps the plant grow more foliage, which produces more sugars to feed the bacteria.
• Soil-applying N at the R3 - beginning pod - stage can assure N is available when the soybean plant needs it the most at R5.5. R3 is when farmers see the first pods forming. The pods are about 3/16 of an inch long. This is the time to apply N if this is a farmer's only opportunity to do so. If using UAN or urea and no rain is in the forecast, use Agrotainª, a urease inhibitor that delays the conversion of urea to ammonia which volatilizes.
• Applying N through irrigation - 20 lbs/A of actual N each at R3, R4, and R5.5.

Role of Precision Farming Tools
Agri-Growth has studied N on soybeans, using precision farming techniques. Researchers can now study concepts and products across entire fields, not just small plots, by using a yield monitor, GPS, and GIS. With this technology, the data input and analyses take much less time than other methods. Also, GIS allows for many types of interpretation, from yield maps to bar charts. In addition, GIS allows for evaluating the impact of treatments against field variability.
    In small plots, hand harvesting and small plot combines provide the greatest accuracy. On 20-acre and larger fields, however, yield monitors can be just as accurate and are certainly more efficient.    Calibration is the key to yield monitor accuracy. Calibration differs by manufacturer, but a general rule is to calibrate for grain weight three to four times with each crop and hybrid or variety to account for different grain flows and combine speeds.
    Expect a few "far-out" readings when running a yield monitor. On a 25-acre field, five out of 8,000 yield readings can be errors. Thus, the errors would represent less than .01 of an acre on a 25-acre field. People experienced in examining yield monitor data can easily filter out erroneous readings. To create meaningful data sets, Agri-Growth uses computer models based on staff experience.

Research Results
In one 1997 research field, Agri-Growth planted soybeans at 54 lbs/A over 20 acres. Researchers applied 60 lbs/A of actual N in strips across the field at R3. They used a broadcast application of urea granules, which were washed in by rainfall. Yield monitor results showed that the fertilized strips yielded 5.6 bu/A more than the un-fertilized strips.
    Farmers can adapt a set of protocols for working their fields by matching the width of the strips to the width of their application equipment.
    Yield maps are valuable tools for comparing with different layers, such as soil types and pH levels. However, the map by itself can be difficult to interpret. For example, one does not see strips of green for N-treated areas and strips of orange for control areas. The mapping software assigns a range of colors for yield levels and there may be no pattern related to the treatments.
    A better way to analyze the data is to use math and graph the results.

Other Research Results
In a 1996 Agri-Growth study, a nitrogen with Agrotain treatment increased yield by 6.6 bu/A.
    Agri-Growth is not alone in studying the concept of N on soybeans.
    In a New Jersey study by soils specialist Roy L. Flannery of Rutgers University, 50 lbs/A N applied at early flower and at early pod fill yielded an amazing 81 bu/A - 21 bu/A more than the check strip. (1990. "Soybeans respond to nitrogen." Agrico Newsletter. Vol. 21, No. 2.)

Growers Try It Themselves
In soybean workshops across the United States, Dr. Ladlie challenges farmers to try N on soybeans. An Ohio farmer took the Agri-Growth challenge in 1997 and tested nitrogen with Agrotain on soybeans. He applied urea with Agrotain at a rate of 55 lbs/A of actual N at the early R3 stage. He left a 1.2 acre check strip down the middle of the 60-acre field. The fertilized soybeans yielded 7.2 bu/A more than the check strip.
    With an ROI that high, this grower is planning to try nitrogen with Agrotain on soybeans again in 1998.
    A grower in Iowa also tested N on soybeans in a 1/4 acre plot in 1997. He applied 100 lbs/A urea broadcast (46 lbs/A actual N) at the R5 stage during rainfall. While this was probably too late for optimal results, he decided to wait for sure rain because he didn't want N-loss to be a variable. The three fertilized strips yielded 10.52 bu/A more than the two non-fertilized strips.
    Ladlie encourages farmers to try this concept of N on soybeans by planting their own test field.
    In 30-inch row soybeans, farmers can use a spinner to apply urea granules or soil-apply liquid UAN with drop tubes. In solid seeding, leave tram lines for application equipment or use aerial applications. If no rain is in the three-day forecast, Ladlie recommends using Agrotain to minimize any N loss. In irrigation, add UAN to the water.
    The timing cannot be stressed enough. Research has shown that preplant applications also help yield, but later applications are more beneficial. If applying N only once, the soybean field should be at R3 - when pods are starting to develop from the nodes that flowered first.

Conclusion
Agri-Growth is studying nitrogen on soybeans for the third year at its research farm in southern Minnesota. Researchers will use tissue testing, infrared photography, yield monitors, and quality sensors to gather data. Growers throughout the United States are also testing this concept and will send their results to Agri-Growth.
    Ladlie said research is needed on varietal response to nitrogen; nitrogen forms, rates, application methods, and timing; and the impact on seed oil, protein, and quality.
    "Growers are looking for ways to increase yields, which have reached a plateau for many. They're dealing with more diseases, soil compaction, and herbicide stress. Growers can manage soybean crops throughout the season. In high yield situations, they can maximize the soybean plant's potential by supplementing N," Ladlie said.

About the authors:
Dr. Jim Ladlie is president of Agri-Growth. He holds a Ph.D. in Crop Science from Michigan State University. In 1979, Dr. Ladlie founded Agri-Growth, which provides training, publishing, marketing, and research for the ag industry. His areas of expertise include training, crop growth and development, research, and marketing. John Oolman is manager of Concept Research for Agri-Growth. He holds an M.S. in Agronomy from South Dakota State University. His areas of expertise include precision farming technologies, soil science and fertility, and concept research.

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