Bacteria soil is really low in nitrogen, you

Bacteria that change nitrogen gas from the atmosphere into solid nitrogen usable by plants are called nitrogen-fixing bacteria.

These bacteria are found both in the soil and in symbiotic relationships with plants.

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What Are Nitrogen-Fixing Bacteria?

Nitrogen-fixing bacteria are microorganisms present in the soil or in plant roots that change nitrogen gases from the atmosphere into solid nitrogen compounds that plants can use in the soil. That’s a mouthful! Let’s break this concept down.

Who Fertilizes the World’s Plants?

It’s spring again, and you find yourself standing in the gardening section of your local home improvement store, looking for the very best fertilizers. You know those flowers in your front yard need a good source of nitrogen to grow big and strong, so you open your wallet for a big bag of Miracle Grow each and every spring.But wait! Who buys the fertilizer for those endless fields of wild flowers, those rolling hills, or those vast expanses of rain forest? Surely all the plants in the world need a good source of nitrogen, right? So where do they get it? Since the Miracle Grow company hasn’t been around since plants began to evolve, something else had to take up the slack. Enter bacteria.

From Air to Soil

Nitrogen is commonly found as an inert, or nonreactive, gas in the earth’s atmosphere. In fact, 78% of the earth’s atmosphere is nitrogen in its gaseous form. This element is so plentiful that it has been incorporated into all living things on Earth. In order for this to happen, though, that nitrogen gas needs to turn into something solid that can be used by living things.A group of bacteria called nitrogen-fixing bacteria take on this challenge. These bacteria take nitrogen from the air and make it react with other compounds and make it solid in the process. This process of taking away the gaseous form of nitrogen and turning it solid is called nitrogen fixation.

Two Forms of Bacteria

Have you ever worked at a large company? If so, you know that different jobs are done by different people. The same thing goes for the nitrogen-fixing bacteria. There are two major forms: free-living bacteria, which live throughout the soil, and mutualistic bacteria, which live in nodules in the roots of certain plants like beans and peas. These two types of bacteria are responsible for fixing 90% of the nitrogen on Earth.


There are lots of species of bacteria that do this job. The free-living, or non-symbiotic, bacteria are made up of cyanobacteria, which most people know as blue-green algae.

Some free-living bacteria also fall into certain genera of bacteria, like the genus Clostridium or the genus Azotobacter. In soils where these bacteria are prevalent, plants will get plenty of nitrogen.There are some soils where you don’t find Azotobacter or Clostridium, however, so these plants need to rely on the other group of bacteria for nitrogen fixation: the mutualistic, or symbiotic, bacteria. These bacteria burrow their way into the roots of plants and form bacteria-filled nodules. Bacteria in the genus Rhizobium are associated with leguminous plants, or plants that bear seeds in pods.

You have probably grown some of these yourself, such as alfalfa, beans, clover, peas, and soybeans.So in areas where the soil is really low in nitrogen, you can plant beans or peas, and the bacteria in the plant roots will take nitrogen from the air and make it solid in the soil, where plants can use it for years to come. I bet you didn’t know that when planting your summer peas, you were doing all your other plants a big favor!

Lesson Summary

Let’s review.

Nitrogen-fixing bacteria are microorganisms that take nitrogen from the air and make it solid. Two types of bacteria do this job: non-symbiotic bacteria in the soil and symbiotic bacteria that live in the roots of plants. Bacteria in the genera Clostridium and Azotobacter are non-symbiotic nitrogen-fixing bacteria, while those in the genus Rhizobium are symbiotic bacteria.

Learning Outcomes

After you’ve reviewed this video lesson, you should be able to:

  • Define nitrogen-fixing bacteria
  • Explain the process of nitrogen fixation
  • Recall how much of the earth’s atmosphere is made up of nitrogen
  • Describe the two types of nitrogen-fixing bacteria and identify examples of each

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