Louisiana Soybeans: What to Know About Nitrogen Fixation

Nitrogen deficiency showing up as lighter green patches in soybeans. Photo: Jon Charlesworth, Purdue University

There have been recent concerns about possible conditions leading to deficient nitrogen fixation in soybean plants. A planting date trial at the Dean Lee Research Station was established on March 30, 2020. It has been between five to six weeks since planting; therefore, this is a good time to evaluate for active nitrogen fixation.

Soybean plants are able to fix up to 70% to 75% of their nitrogen requirement by converting atmospheric nitrogen gas (N2) into ammonium (NH4+). This nitrogen fixation process is from a symbiotic relationship between the soybean plant and Bradyrhizobium japonicum bacteria colonies living on the soybean root.

The colonies appear in the form of nodules attached to the root. The remaining nitrogen requirement of the soybean plant is usually available from the soils, 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 that were not planted to soybean for the previous three to five 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 are coarse-textured, compacted, have low or high pH, contain high residual soil nitrogen levels or are flooded for three or more days may negatively affect the nitrogen fixation process.

In low-pH soils, nitrogen fixation is negatively affected due to the low availability of molybdenum (Mo), a key component of the nitrogenase enzyme that drives the biological nitrogen fixation process. Poor nodulation may also result from unhealthy soybean plants that cannot supply an adequate amount of carbohydrates to the bacteria.

Soybeans should be planted in fields with a soil pH around 6.5. If soybeans are planted in fields below 6.0 pH, molybdenum should be applied as a seed treatment. However, molybdenum should not be used as a seed treatment with Rhizobium inoculum unless planting immediately after treating the seed.

Fields with soybean plants showing nitrogen deficiency (short and pale green to yellow plants with non-prominent veins) should be evaluated for poor nodulation by digging up and washing the roots. At the V3 to V5 growth stages, at least seven nodules (2 millimeters or greater in size) with a pink or red colored cross-section should be found in each plant. If the plants show nitrogen deficiency, applying supplemental nitrogen may be economical.

If applying nitrogen, use granular/dry nitrogen fertilizers that would reduce foliage burning. The rate of supplemental nitrogen required depends on soybean growth stage, average yield history and available nitrogen from the soil. Usually, soybeans remove about 4 pounds of nitrogen per bushel of grain harvested.

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Figure 1. A soybean plant with nitrogen deficiency symptoms. The limited quantity of nitrogen in the plant is translocated to newer leaves, leaving older growth with a pale-green to yellow color. Photo by the University of Nebraska-Lincoln

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Figure 2. Bradyrhizobium japonicum bacteria nodules attached to the roots of a soybean plant at the V3 growth stage. A penny gives perspective to the size of the nodules.

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Figure 3. A cross-section view of a Bradyrhizobium japonicum bacteria nodule from a soybean root. The cross-section view of the nodule has a pink or red color, suggesting the nodule is healthy and actively fixing nitrogen. A penny gives perspective to the size of the nodules.

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