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Bacteria give legumes their high protein content

By Harold Keyser


Legumes are known for their high protein content and the symbiosis they form with rhizobia bacteria, which provide them with free nitrogen from the atmosphere.

Legumes, the third-largest family of plants, provide us with many options for our gardens, including high-protein vegetables, ground covers, "green" manures, windbreaks, shade, living fences and forages for livestock.

You can learn more about legumes and the many choices for use in your garden:
» www.ctahr.hawaii.edu/bnf
» www.ctahr.hawaii.edu/sustainag/cc-gm

The basis for their high-protein content is found in their root nodules, where rhizobia can use otherwise inert nitrogen gas in the air and convert it to useable ammonia through a unique nitrogenase enzyme. The plant feeds the bacteria in the nodule in exchange for the "fixed" ammonia. The plant does the rest, converting the ammonia to amino acids and protein. This symbiosis can provide the legume with its entire nitrogen requirement.

Beans, peas, peanuts and soybean are the common vegetable legumes found in Hawaii gardens. These four legumes each require different rhizobia for an effective symbiosis, and their presence in any given soil depends on many factors.

A simple test performed before planting can help you determine whether the specific rhizobia are present: Put a small amount of your garden's soil in a pot and plant the legume of interest. Examine it for nodules after about three weeks and cut open the nodule. Look for a pink to red color, indicative of active nitrogen fixation. If nodules are lacking or the cut-open nodule is white, legume inoculant containing the specific rhizobia is required. The inoculant can be obtained from nurseries or seed supply stores.

To take full advantage of the benefits from legumes, having the compatible rhizobia is necessary.

Question: I have heard that legumes increase the fertility of soil. What are my options for doing this?

Answer: One of the best options is to grow the appropriate legume as a green manure that gets incorporated into the soil. As a regular practice, this will increase the organic matter, nitrogen content and water-holding capacity; improve soil structure and aeration; and reduce the potential for erosion. Candidates include Sunn hemp, pigeon pea, white sweetclover, lablab and Stylosanthes guianesis, or stylo. Theses legumes can also be used as a feed source for some livestock.

Q: Is there a legume I could use as a cover crop in my backyard orchard?

A: Legume cover crops can benefit the soil and a perennial crop at the same time. An excellent candidate is perennial peanut with coffee, banana or citrus. Other candidates include Desmodium triflorum, or tick clover, and Stylosanthes guianesis, or stylo.

Q: I have heard that mulch made from legumes is like a slow-release fertilizer. What legumes are best for this purpose?

A: Some woody legumes offer their own variety of uses, including Gliricidia sepium, commonly known as gliricidia, and Leucaena leucocephala, the common invasive weed known as koa haole, in your garden. They can be repeatedly pruned, with their high nitrogen foliage laid as mulch. Gliricidia can also be spaced for use as a living fence or windbreak and to provide shade.

Harold Keyser is the Maui County administrator for the University of Hawaii's College of Tropical Agriculture and Human Resources.

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