Main Function of Nodules in Plants

Beneficial Bacterial

• The nodules on the roots of legumes house microscopic bacteria that produce nitrogen for the plant to use. They are called Rhizobium bacteria and they have a mutually beneficial relationship with the host plant. The host provides energy and nutrients for the bacteria, which in turn synthesizes nitrogen for the host plant to use. Rhizobium are present in most soils, but gardeners can also inoculate legumes seeds prior to planting to ensure efficient nitrogen fixation.

Nitrogen Fixation

• Nitrogen gas comprises 80 percent of the earth's atmosphere, but this not a form that plants can assimilate. Nitrogen-fixing bacteria have the vital role of converting atmospheric nitrogen into a soluble form that plants can use -- a process that makes life possible for all other plants and animals. Nodules are white or grey in color when they first appear a few weeks after the plant has germinated and do not make any nitrogen at first. As the host plant grows, the nodules swell in size and turn pinkish-red. The deeper the color, the more nitrogen they are making.

Plant Sustenance

• Nitrogen is the limiting nutrient for green vegetative growth, so the nitrogen-fixing nodules give legumes a huge advantage over other plants. In nature, legumes are often found on soils of naturally low fertility, such as an area of upturned, eroded earth after a landslide. The conversion of atmospheric nitrogen in the root nodules sustains the plants in environments where other species fail to thrive. The accumulation of plant material in the soil in these depleted environments eventually improves growing conditions, allowing other species to become established.

Natural Fertilizer

• The nitrogen accumulated by root nodules can be anywhere from a few pounds per acre to several hundred pounds per acre. The amount varies according to the species, the presence of sufficient Rhizobia bacteria, and environmental conditions. Most of the nitrogen is used by the legume for its own growth, but some is returned to the soil when the plant dies, and this nitrogen becomes available to other species. In this way, nodules function as a natural producer of fertilizer -- a valuable trait that wise gardeners learn to take advantage of.

Cover Crops

• Fall- or spring-planted "cover crops" are the most common way that gardeners use legumes to enrich the soil. To get the most nitrogen out of the planting, the legume needs to grow until it begins to flower. Once the majority of the flowers have opened, the cover crop is cut to the ground, releasing the accumulated nitrogen into the soil, plus a good quantity of organic matter. A crop can then be planted to make use of the nitrogen generated by the root nodules.