La. Soybean Crop Progress & Condition Week Of June 29

Dr. David Mosely

LSU AgCenter

As of June 28, a total of 72% of the Louisiana soybean crop was blooming and 30% was setting pods. 100% of the crop was rated fair-excellent, with 82% rated good-excellent. The rating of good-excellent was up 9% from the previous week and is the highest rating of the season (. It was reported a soybean field in St. Mary Parish has entered the R7 growth stage where at least one pod on the main stem has reached its mature color (). Last week there were rainstorms throughout the state. Hopefully, the rain supplied a timely and helpful amount of moisture as the plants are reaching the reproductive stages.

In March, the USDA projected Louisiana producers intended to plant 980,000 acres of soybean in 2020 with 83.5 million intended soybean acres across the United States. Both projections would have been a 10% increase from 2019. On June 30th, the USDA reported the number of acres across the United States may remain consistent with the March projection; however, Louisiana producers are projected to plant 1.1 million acres of soybean, a 24% increase from 2019.

A total of 82% of the Louisiana soybean crop is rated good – excellent.

Evaluating Nitrogen-Fixing Nodules

This week, I visited a producer concerned of an insufficient amount of active nitrogen-fixing nodules. Soybean plants are able fix most of their nitrogen requirement through a symbiotic relationship with Bradyrhizobium japonicum bacteria. The bacteria form nodules on soybean root hairs and convert nitrogen gas (N2) into ammonium (NH4+), a form usable by the plant. However, there are several reasons that can cause soybean plants to be deficient in nitrogen, including an insufficient number of active nodules.

If the plants have symptoms consisting of light green to yellow leaves, they may be deficient in nitrogen. However, a deficiency in sulfur can cause very similar symptoms. A tissue sample can be helpful to determine if the plants are deficient in one or more nutrients. Another good visual is looking at the nodules attached to the roots, but care should be taken to remove the plants with a large amount of soil still intact around the roots, and then the soil should be gently washed from the roots. If the plant is pulled from the soil or if the soil is shaken off the roots, many of the lateral roots and the attached nodules will fall off (). If the soil is washed from the roots, the lateral roots and nodules should remain (). If the plants are deficient in nitrogen from a lack of nodulation, an application of nitrogen can be economical. Ideally, the application should be applied between the R1 and R2 growth stage. However, applications at R3 have also been reported to show an increase in yield. If the plants have a nitrogen deficiency for another reason, such as a lack of oxygen in the soil, then applying supplemental nitrogen may not have a positive effect on yield.

Soybean plants with light green to yellow foliage suggesting a nitrogen or sulfur deficiency.

The LSU AgCenter Dean Lee Research and Extension Center will hold a virtual field day. Research presentations by faculty will begin July 20, 2020. More information can be found at https://www.lsuagcenter.com/portals/our_offices/research_stations/deanlee/features/dean-lee-virtual-field-day