Diagnostics: Top Dressing Nitrogen on Soybeans

There is a lot of interest in applying nitrogen on soybeans as more growers are willing to go to greater lengths to improve soybean yields on their farm. There are many opinions on whether it works or not—some data show a positive response, but other data suggest no response. And every agronomist and expert seems to have their own opinion. The fact that nitrogen responses are inconsistent and unpredictable makes this decision complicated and risky and never as sure a thing as it is with corn.

Soybeans need nitrogen and plenty of it—from 4.5 to 5.0 lbs. per bushel, so a 60 bushel crop will need 300 lbs. Because soybeans are legumes, they can fix enough nitrogen under ideal conditions to feed a 50-bushel crop, and the soil will usually supplement the rest—maybe another 20 to 40 lbs. depending on soil conditions and organic matter. Remember that the same soil conditions that promote good nodulation and nitrogen fixation also promote nitrogen mineralization.

This season, many soybeans didn’t nodulate in May and June due to excessive rain and poor soil conditions. Not only did soybeans not nodulate, but soil nitrate was leached below the root line, and nitrogen mineralization was reduced as well. So soybean growth was stunted, and plants took on a yellowish hue due to lack of rooting activity and poor nitrogen fixation.

So what is a grower to do when faced with yellowish and stunted beans that have fewer nodules, at the same time as plants are setting and beginning to fill pods? Growers can top dress nitrogen on their soybeans. Making a nitrogen recommendation may not be precise or predictable; however, if plants lack nodules, excessive rain leached nitrate out of the soil and soybeans show deficiency symptoms, this is the season to give it a try.

University research has shown that fields with no history of soybean production in the past couple of years require an additional 50 to 60 lbs. of nitrogen per acre for good yields, even when you inoculate the seed at planting. This is a good recommendation as rescue treatment. If nodulation is questionable and plants have deficiency symptoms, apply enough nitrogen to bring the soil level up to 60 lbs. actual nitrogen per acre. That means doing a soil test to look for residual nitrate in the top 24 inches of soil and then making up the difference with commercial fertilizer.

At this point, the only option is top dressing nitrogen with a broadcast spreader or aerial application when nitrogen deficiency symptoms are obvious. June is too soon to apply  nitrogen as it may be lost when excessive rain falls or from ammonia volatilization if no rain falls. Soybeans nitrogen demand increases greatly at R5, when plants are filling pods (building seeds), so to be in front of this demand, plan to top dress at R3 or R4. Dribbling UAN on the soil surface is an option if the rows aren’t closed over, but by R4 it is probably too late do that.

For more information on applying nitrogen on soybeans, read this brief overview from the International Plant Nutrition Institute (IPNI).

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agrwrite.com