Md Rasel Parvej, Moseley, David O
Dr. Rasel Parvej, LSU AgCenter Soil Fertility Specialist and Dr. David Moseley, LSU AgCenter Soybean Specialist
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 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 problem 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 N per acre 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. 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. A 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) in their high yielding fields during R3 to R4 stages. However, the following conditions should be considered when deciding if supplemental N may be beneficial.