Nitrogen Fixing Bacteria Rhizobium

Microbes are the oldest form of life on earth. They first appeared about 3.5 billion years ago and critically important in sustaining our life on earth. Microbes generate at least half of the oxygen we breathe. In 1885, the Dutch microbiologist, discovered another critical function they performed – Fixing nitrogen.

Approximately, 80% of the atmosphere is nitrogen gas. It is too stable for the plants to break down the nitrogen, that’s where the microbes come into picture. Since, N2 is not used by most living organisms, plants, animals, and micro-organisms die of nitrogen deficiency, surrounded by the N2 they can’t use. That’s because they use the ammonia form of Nitrogen.


These microbes help fix the nitrogen that is one of the building blocks of all of our body and all of our biochemistry. All plants depend upon the nitrogen and also human beings as the food that we eat has been prepare with the help of the nitrogen that has been fixed by the bacteria.

The Bacteria that helps breaking down the nitrogen are called Rhizobium. These bacteria live with close association to plants, in legumes and also in a few other plants, the bacteria live in small branches on the roots called nodules. In these nodules, nitrogen fixation is done by the bacteria. For this procedure, the plants and microbes have a symbiotic relationship. Some effort is needed by the plant as well in this procedure.

Rhizobium live in intimate association of pea plants (alfalfa, beans, clovers, peas, soybeans)


If a plant has been planted newly and is growing slowly, then you can suspect that there are insufficient nitrogen fixation. This is commonly seen with beans and alfalfa. Poor fixation is attributed to the lack of native Rhizobium to nodulate the legume, but the cause can also be poor plant nutrition and other plant stresses that inhibit nitrogen fixation. Generally small nodules are going to be present after 2-3 weeks after germination.

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