Early Nodulation Checks Can Reveal Yield-Limiting Nitrogen Gaps in Soybeans
Maximizing soybean yields starts with meeting the crop’s nitrogen needs – with soybeans requiring 3.5 pounds of nitrogen per bushel of grain, strong nodulation and nitrogen fixation are critical for performance.
Soybean roots form nodules when rhizobia bacteria in the soil colonize root tissue. In this symbiotic relationship, the plant supplies energy in exchange for the bacteria “fixing” nitrogen from the air into a usable form for the plant.
“At [V1 or V2], I’m not overly worried about the top growth of this plant,” said Jonathan Rotz, Pioneer strategic account manager. “What I am really going to look at is what is going on below ground. One of the first places we want to look is the nodulation.”
To check nodulation, dig up a soybean plant and look for nodules, which appear as knot-like structures on the roots. By the V2 or V3 growth stage, plants should have seven to 14 nodules that are pink or red when split open.
Several conditions can reduce nodulation, including:
- Low rhizobia bacterial levels in the soil
- Lack of oxygen due to soil compaction or over-saturated soils
- Rhizobia source applied to the seed was not viable at planting time
- Drought stress
- High availability of nitrogen in the soil
Reduced nodulation can cause nitrogen deficiency symptoms. If plants have five or fewer nodules for two weeks at the V2 or V3 growth stage, or if nodules appear green, white or brown when split open, nitrogen fixation may be inadequate. In this case, supplemental nitrogen may be needed.
Supplemental Nitrogen Application Recommendations
Applying less than 44 pounds of actual nitrogen is recommended as larger applications do not increase yield.
It is best to apply urea treated with a urease inhibitor. Liquid nitrogen sources, such as 28% UAN, can cause significant leaf burn to the crop canopy, so pay special attention to the application area.
If possible, band between rows to improve plant access and uptake. This may be harder in drilled or 15-inch soybean rows as there is a higher chance of damaging plants with each pass.
Applying the fertilizer at early flowering, when foliage is dry, improves nitrogen uptake as soybeans begin moving nitrogen from vegetative tissue to the grain. This timing supports grain production and can extend the period the plant stays green without reducing nodulation.
A High-Yield Practice for the Future
Research shows yield is often limited by disease, nematodes or drought stress, but outside of these limitations, soil and fixed nitrogen becomes limiting at average yield levels between 60 and 80 bu/A. As higher soybean yields become more common due to improved genetics and management practices, additional nitrogen may be needed to maximize yield potential.
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