Nitrogen (N) is the most yield limiting nutrient for any row crop including soybean. Since soybean, like other legume crops, can fix atmospheric N symbiotically through Rhizobium bacteria (Bradyrhizobia japonicum), the crop usually does not require any supplemental N.
However, symbiotic N fixation only supplies 20 to 80% of the total N requirement to the soybean plant (depending on soil types, soil N concentration, cultivars, and management practices).
When discussing N fixation in soybean, a common question is usually asked: Does soybean need supplemental N to maximize yield? Unfortunately, there is no straight answer for this question. Researchers have studied soybean response to N fertilization over 200 environments across the United States, but the results are mixed: either no response or an increase or decrease in yield.
The reason for decreased yield or no response is mostly because supplemental N can suppress symbiotic N fixation through poor nodulation. High residual soil N also can decrease nodule production and reduce N fixation. Most researchers that found soybean yield decline or had no response to N fertilization applied their N fertilizer treatments at planting or a few days after planting.
Therefore, it is almost certain that under ideal conditions any supplemental N at or a little after planting would not give any yield benefit unless there is a deficiency in N fixation caused by ineffective Rhizobium populations or other soil and environmental conditions (e.g. high residual soil N concentration, drought, waterlogged soils, etc).
However, soybean will respond to preplant N fertilization if the symbiotic N source is completely replaced the by inorganic N fertilizer. This often happens in yield contest fields where producers try to yield >100 bushels soybean per acre partially by applying an abundance of N (500 pounds or more) and do not rely on any N from the N fixation process. However, this is not an economically viable soybean production practice.
Researchers who found a soybean yield benefit from supplemental N, applied their N fertilizer treatments during the reproductive stages at or after R3 (pod set) for high yielding irrigated soybean.
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The reason for a soybean yield increase from late-season N application is partly due to the inadequate supply of N from symbiotic N fixation (inactive nodules). A soybean plant N demand peaks from the R3 to R6 stages when soybean requires N from both soils and the nitrogen fixation process: especially in high yielding irrigated fields.
Does N fixation supply enough N during these reproductive stages to maximize soybean yield under high yielding conditions? Again, researchers have mixed opinions for this question. One of the strong opinions is N demand during pod-set to seed-filling periods may be greater than N supply from symbiotic N fixation under high yielding irrigated conditions and supplemental N is required to maximize soybean yield.
Note that soybean can produce around 80 bushels per acre by utilizing symbiotically fixed N under high yielding irrigated conditions. Supplemental N may be needed to produce more than 80 bushels per acre.
Considering this possible yield limitation with only utilizing the N fixation process, we are evaluating soybean response to late-season N application across different environments. The results will be shared at the end of this growing season.
Although the LSU AgCenter currently does not recommend any late-season N application for soybean production in normal conditions, if interested, producers may apply 25 to 45 pounds N per acre (50 to 100 pounds urea: 46-0-0) during R3 to R4 stages. The following conditions should be considered when deciding if supplemental N may be beneficial.
- High yielding fields (average yield >70 bushels) with irrigation and no other nutrients shortage.
- High yielding soybean cultivar. The LSU Agcenter annually publishes results from the LSU AgCenter Official Variety Trial and Core-block demonstrations.
- The crop was planted at the optimum planting window (mid-April to mid-May) to maximize yield potential.
- Coarse-textured soils (sandy to silt loam) with low residual soil N concentration (usually fine soils such as clay have high organic matter and release high amounts of N during the growing season where plants are less likely to response to supplemental N).
- Poor or inactive nodulation. At the V3 to V5 growth stages, there should be at least seven nodules that measure 2 mm or greater. The inside of an active nodule should have a pink or red color. Please read the Nitrogen fixation in soybeans article from the Louisiana Crops Newsletter Volume 10, Issue 4 – May 2020 for more information.
- Crop experienced drought or water-logged conditions during earlier stages.