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Biological Nitrogen Fixation (5/18/17)

Legumes including alfalfa, chickpea, field pea, lentil, soybean and dry bean, have the ability to establish a symbiotic relationship with rhizobia bacteria to biologically fix nitrogen gas (N2) into plant available nitrogen (N).

Biological Nitrogen Fixation

Legumes including alfalfa, chickpea, field pea, lentil, soybean and dry bean, have the ability to establish a symbiotic relationship with rhizobia bacteria to biologically fix nitrogen gas (N2) into plant available nitrogen (N). The fixed N provides part of the crop’s N requirements and most plants will also utilize soil available N for plant growth. Inoculants containing living rhizobia bacteria can be applied in liquid, peat-based powder, or granular form. Increasing the number of bacteria near the roots of the legume may increase the number of nodules per root and therefore improving the N supply to the plant. The various legume crops grown in the region need to be inoculated with the crop-specific rhizobia bacteria (Table 1). Field pea and lentil can use the same inoculum since identical species of bacteria form associations with both pea and lentil. If a legume crop has never been grown in a field before, inoculation with the correct species of bacteria is essential.

 Fields which already have a specific rhizobium population, which occurs when a host crop has been grown in recent years, may not always give a positive yield response to additional inoculation. Since the bacteria in the inoculum are living organisms, high temperatures (for instance seed pre-treated with an inoculum sitting out in the hot sun) may reduce the number of live bacteria.

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Field pea

Once the crop is established, nodules will form on the roots of the legume. About 2-3 weeks after emergence, field pea should begin nodule formation on root hairs. To check N-fixing, open the nodule and look at the color inside the nodule. If the color inside is pink, then the nodule is actively fixing N2 into plant available N. If the inside of the nodule is brown, green or white, the N fixation is not taking place. Nitrogen fertilization in pea (or soybean) is not generally recommended because with excessive soil N available to the plant, the nodulation will be delayed and possibly fewer nodules will be formed. However, if the plants appear pale green or yellow due to lack of N, and the roots do not exhibit nodulation, then N top-dressing may be a rescue option (make sure that the yellowing is due to lack of N and not due to other reasons).

When checking the pea root, also observe if there are diseases present, as diseased roots will have lower nodulation ability. When checking for nodules it would be best to dig up plants with a small spade, wash the roots in a bucket of water and observe the roots. Check at several locations and several plants per sampling site. The principles for checking other crops like lentil, chick pea, or soybean are similar to checking field pea plant roots.

 Hans Kandel

Extension Agronomist Broadleaf Crops

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