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Nodulation

Also known as
root nodules, nodule formation

Nodulation is the formation of root nodules on a legume, such as soybean, by nitrogen-fixing rhizobia bacteria. The nodules house the bacteria and are where atmospheric nitrogen is converted into a form the plant can use.

What is nodulation?

Nodulation is the process by which a legume forms small growths, called nodules, on its roots in partnership with rhizobia bacteria. Each nodule is a specialized organ where the bacteria live and carry out biological nitrogen fixation, turning nitrogen gas from the air into ammonia that the plant uses to grow. Good nodulation is what lets soybeans and other legumes meet much of their nitrogen need without fertilizer.

How nodules form

  1. Signaling. The legume root releases flavonoids into the soil. Compatible rhizobia respond with signals called Nod factors.
  2. Infection. The bacteria attach to a root hair, which curls around them. The plant forms a tube, the infection thread, that carries the bacteria into the root.
  3. Nodule development. Root cells divide to build the nodule, and the bacteria move into those cells.
  4. Nitrogen fixation. Inside the nodule the bacteria change form and begin fixing nitrogen. The plant supplies them with sugars and produces leghemoglobin, a red protein that controls oxygen levels so the nitrogen-fixing enzyme can work.

In soybeans, nodules begin forming within a few weeks of emergence and fixation increases through the vegetative stages, peaking during pod development.

How to check soybean nodulation

Checking nodulation tells a grower whether an inoculant worked and whether the crop is fixing nitrogen. Extension guidance generally recommends:

  • Dig, do not pull. Pulling a plant strips nodules off the roots. Dig plants with a spade, keeping a good root ball, and gently wash or shake the soil away.
  • Look at location and number. Nodules on the taproot and upper roots formed early and indicate successful early infection. Check several plants from different parts of the field.
  • Cut nodules open. An active nodule is pink to red inside, from leghemoglobin. White, green or brown nodules are not fixing nitrogen, either because they are too young, damaged, or no longer active.

What affects nodulation

  • Rhizobia present. Fields that have not grown the legume recently, or whose rhizobial populations have declined, need a microbial inoculant.
  • Inoculant viability. Rhizobia on seed can die from heat, drying, delays before planting, or contact with incompatible seed treatments.
  • Soil nitrogen. High levels of soil or fertilizer nitrogen suppress nodule formation and fixation, because the plant uses the easier source.
  • Soil conditions. Waterlogging, drought, very acidic soils and high soil temperatures all reduce nodulation.
  • Root health. Root disease, compaction and injury reduce the root surface available for infection.

Nodulation and yield

Poorly nodulated legumes rely on soil nitrogen, which may not meet the crop’s needs at high yield levels, and often show pale, yellow-green leaves typical of nitrogen deficiency. Strong, early nodulation supports nitrogen supply through the pod-filling stages, when demand is highest.

Nodulation at Locus

Locus positions its soybean inoculants, including a planter-box product and a multi-strain system, around consistent early nodulation and reliable nitrogen fixation.

Frequently asked questions

Pink to red. The color comes from leghemoglobin and indicates the nodule is fixing nitrogen. White nodules are usually immature; green or brown nodules are inactive or dead.

Nodules are visible within a few weeks after emergence. Checking during the early vegetative stages shows whether inoculation worked; checking around flowering shows whether nodules are active when nitrogen demand is rising.

Yes. When plenty of nitrogen is available in the soil, legumes form fewer nodules and fix less nitrogen, because taking up soil nitrogen costs the plant less energy than supporting nodules.

  • Rhizobia

    Rhizobia are soil bacteria that form nodules on the roots of legumes, such as soybeans, peas and alfalfa, and inside those nodules convert nitrogen from the air into a form the plant can use. Each legume partners with specific rhizobial species.

  • Biological nitrogen fixation BNF

    Biological nitrogen fixation (BNF) is the conversion of nitrogen gas from the air into ammonia by certain bacteria and archaea, using the enzyme nitrogenase. In agriculture, the most important example is rhizobia fixing nitrogen inside the root nodules of legumes.

  • Bradyrhizobium japonicum

    Bradyrhizobium japonicum is a soil bacterium that forms nitrogen-fixing nodules on soybean roots. It is the rhizobium most commonly used in soybean inoculants. Soybeans are not native to North America, so fields without a history of soybeans may lack enough of the bacterium.

  • Double inoculation

    Double inoculation is applying rhizobia inoculant to soybeans twice, or by two methods at once, such as a seed-applied product plus an in-furrow or planter-box product, to increase the number of nitrogen-fixing bacteria near the young roots.

  • Microbial inoculant

    A microbial inoculant is an agricultural product containing live beneficial microorganisms that is applied to seed, soil or plants so the organisms can establish and benefit the crop. Rhizobia inoculants for legumes are the oldest and most widely used example.

Sources

  1. Chapter 23: Nitrogen fixation, in iGrow Soybeans, South Dakota State University Extension
  2. Soybean inoculation and nitrogen fixation, Science for Success, soybean extension specialists
  3. Revisiting biological nitrogen fixation dynamics in soybeans, Peer-reviewed review, PMC8529188