One day the soybean field on your way from home to the grain center is green. The next time you notice it a few days later, it is turning yellow. All that means to you is that the soybeans are starting to mature, and it’s time to speed up the process of getting the combine ready to roll.
Was there something else you could have learned if you had stopped by that field and walked a bit, even a couple of times, between the green stage and the yellowing stage? Steve Gauck thinks there was. You might not have learned anything that would help this year’s crop, but you might have unearthed valuable knowledge that would guide your decisions and put soybeans in a position to yield more next year.
Gauck is a sales agronomist for Beck’s, based near Greensburg, Ind. Beck’s is the sponsor of Soybean Watch ’17.
One of the things Gauck looked for when he inspected the Soybean Watch ’17 field a few weeks before harvest this year was what plants were telling him about the use of nitrogen made by nodules on the roots late in the season.
Some plants were still healthy and green. Others were starting to show symptoms of yellowish leaves. This was about five weeks before the field would be ready to harvest. Gauck dug up roots of both plants that were still green and those that showing signs of starting to yellow.
“One plant I checked that was still green had quite a few nodules left when I carefully dug up the plant,” he says. “You hope that the nodules out near the root tips are still pink when you open them. That means they are still working, still fixing N from the air and making it available to the plant to continue to make sugars through photosynthesis and deposit those products in developing soybeans in the pod.”
Some plants that had a few yellow leaves had fewer nodules when Gauck dug them up. “We had a fairly dry August here, and it’s possible that dry weather stress caused some nodules to dry up,” he says. “Even if the soil type is only a bit higher, it may have made enough difference in soil moisture levels, and the plant began running out of nitrogen earlier.”
Some growers have tried adding a nitrogen application around V4 to keep N available to plants longer, especially beyond when nodules shut down, Gauck notes. The results have been inconsistent so far, he says. Sometimes a late-season N application helps, and sometimes it produces little or no benefit — definitely not enough to cover the cost.
Gauck did make one observation that might lead to a future decision. “This field was hit with a huge rain, and there was ponding. Some beans were lost, leaving bare areas,” he says. “You might think about using inoculant the next time it’s in beans. Ponding and flooding tends to kill off nitrogen-producing bacteria.”
Check out the slideshow for a look at how nodulation affects soybean health.