Edamame (Glycine max) are the immature pods of a variety of soybean. Like most legumes, they have a relationship with bacteria (Rhizobium spp.) in the soil that allow them to accumulate nitrogen and make it available to other plants growing nearby. You can inoculate edamame seeds with the microbes before planting, though this is not essential.
Nitrogen-Fixing Soil Bacteria
Nitrogen is the primary nutrient responsible for green vegetative growth in plants. It is abundant in the atmosphere as a gas, but in soils it exists in limited quantities in the soluble form that plants are able to use. The process is called nitrogen fixation, and the bacteria that are responsible for it live naturally in most soils. Inoculating edamame seeds before planting ensures a large population of the bacteria is present as soon as the plant begins to grow.
Reduced Edamame Growth
If the seeds are not inoculated, the growth of edamame may be reduced. Edamame does not require nitrogen-fixing bacteria to grow, but the plants are more vigorous with this built-in source of nitrogen. If large populations of nitrogen-fixing bacteria are already present in the soil, there may be no need to inoculate. This is often the case in rich garden soils that have been cultivated where legume crops have already been grown. Newly cultivated areas may have only small quantities present, or none at all. In this case, fertilizer can be used as a source of nitrogen for edamame. If the seeds are not inoculated and bacterial colonization is poor, the plants can also be inoculated in the ground. Simply dilute the powdered inoculant and drench the soil around the plants.
Cover Crop Performance
One reason for inoculating edamame is to enrich the soil for other plants, a technique called cover cropping. After the pods have been harvested, the remains of the edamame plants can be cut to the ground and tilled in to the soil. When the roots die, all the nitrogen stored in the nodules is released into the soil and can be used by subsequent plantings. Edamame is unlikely to perform well as a cover crop if the seeds are not inoculated at planting time.
It is easy to tell if nitrogen-fixing bacteria are doing their work on the roots of edamame. The bacteria form small nodules on the exterior of the roots that can be up to 1/4 inch in diameter. The nodules may appear white or pinkish-red. Good nitrogen fixation shows as large numbers of colored nodules and is usually the result of effective inoculation. In plants that have not been inoculated, there may only be a few whitish nodules.