By Jeremy Ross, Extension Soybean Agronomist
I’ve had several calls over the last few days with producers who had already had corn planted, and due to flooding are considering going back with soybean. The number one question I’ve been asked is “Will the 70-100 pounds of nitrogen that was applied to the corn crop hurt the soybean crop that will be planted back into these fields?” The short answer is the nitrogen applied prior to corn planting will not adversely affect the soybean crop. We can assume that some portion of this nitrogen will be lost due to runoff as the flood waters recede or leached down into the soil profile. The remaining nitrogen will be used by the soybean plant. When it comes to inorganic nitrogen, the soybean planted doesn’t care if the nitrogen comes from native soil nitrogen, fertilizer nitrogen, or nitrogen supplied by the bacteria associated within the nodules on the soybean roots.
When it comes to nitrogen needs, a soybean plant requires around 5 pounds of nitrogen to produce one bushel of grain. A 50 bushel per acre soybean crop will require 250 pounds of nitrogen per acre. Between 50-75% of this nitrogen comes from nitrogen fixation that occurs in the nodules on the soybean roots. High levels of residual soil nitrogen can inhibit nodule formations and activity, but the remaining nitrogen from the application prior to corn planting should be used by the soybean plants quickly, and proper nodulation should occur. Once the residual soil nitrogen is reduced to a low level, the process of nitrogen fixation in the nodules will begin. However, it will take about two weeks for the nodules to produce nitrogen the soybean plant can use.
We have been looking at soybean inoculants for the last several years. Many of our original studies planted early in the planting window (mid-April to mid-May) showed no yield response with the use of inoculants compared to the untreated check. For the last three years, we have been evaluating the effect of planting date and inoculants on soybean grain yield. Results from the last three years are showing a significant yield response with inoculants in delayed plantings. Mid-June planting dates are showing an average of 6 bushel/acre and mid-July planting dates are showing an average of 11 bushel/acre soybean grain yield increase when compared to the untreated check. We have evaluated several commercially available inoculants, and all seed treatment inoculants preformed equally. We did see a slightly lower yield response with the dry, hopper box products compared to the seed treatments. This could be due to better coverage with the seed treatments, thus more bacteria per seed with the seed treatments. Our new recommendation is the use of soybean inoculants for any soybean that is planted after May 15. We would prefer the use of a seed treatment product, but would recommend the dry, hopper box products if this is the only option. Our previous recommendations pertaining to soybean inoculants have not changed.
More information on soybean production, including access to all publications and reports, can be found at http://www.uaex.edu/soybean.
We sincerely appreciate the support for this publication provided by the soybean producers of Arkansas and administered by the Arkansas Soybean Promotion Board.