LA Soybean Crop Progress And Condition

By David Mosely, Ph.D.

LSU Ag Center

Louisiana soybean producers have made significant progress in planting over the previous two weeks. According to the USDA, 68% of the soybean crop had been planted by May 10 with 46% emerged. The 2020 soybean planting season continues to be ahead of 2019. The progress of the 2020 planting season was even with the 5-year average until April 26 but is now tracking slightly faster. Statewide, the soybean crop is rated as 96% fair to excellent.

The 2020 Louisiana soybean planting progress is ahead of the 2019 planting season as of May 10 and is tracking slightly ahead of the five-year average.

There have been reports that certain soybean production fields have been negatively affected by heavy precipitation during the last week of April, with some fields possibly requiring a replant. Information for replanting decisions can be found in the Louisiana Crops Newsletter. An article, suggesting stand counts below 70,000 to 75,000 plants per acre may result in lower yields, was published in April 2020. Besides potential increase in yield, the economics of replanting should be considered. There will be an article in the May edition of the Louisiana Crops Newsletter covering information on the cost analysis of replanting soybean.

Recently, there have been concerns on possible conditions leading to deficient nitrogen-fixation from the soybean plants.

Nitrogen-fixation

Soybean plants are able to fix up to 70-75% of their nitrogen requirement from converting atmospheric nitrogen gas (N2) into ammonium (NH4+). This nitrogen-fixation process in from a symbiotic relationship between the soybean plant and Bradyrhizobium japonicum bacteria colonies living on the soybean root. The colonies will appear in the form of nodules attached to the root. The remaining nitrogen requirement of the soybean plant is usually available from the soil, with no supplemental nitrogen required. Furthermore, applying supplemental nitrogen to soybean plants can decrease the amount of nitrogen fixed through the symbiotic relationship. The seed should be inoculated with Bradyrhizobium japonicum bacteria when planting into fields not planted to soybean for the previous 3 – 5 years, and in fields that were previously under sustained flooded conditions. Unfortunately, the symbiotic nitrogen-fixation process can fail under certain environmental conditions. Soils that contain a coarse-texture, are compacted, have a low or high pH, contain high residual soil nitrogen levels, or are under flooded conditions for three or more days, may negatively affect the nitrogen-fixation process. Poor nodulation may also result from unhealthy soybean plants that can not supply an adequate amount of carbohydrates to the bacteria.

Fields with soybean plants showing nitrogen deficiency (short and light-green or yellow plants) should be evaluated for poor nodulation by digging up and washing the roots. At the V3-V5 growth stage, at least seven nodules (2 mm or greater in size) with a pink or red colored cross-section, should be found per plant. If the plants have poor nodulation, applying supplemental nitrogen may be economical. If applying nitrogen, the application should be made timely and with a method that will not burn the foliage.

An article on nitrogen-fixation has been submitted for the May edition of the LA Crops Newsletter. Please see the newsletter for more information.