The Nitrogen Cycle

All Life depends upon the chemical element nitrogen.

An atom of nitrogen lies at the heart of all amino acids, which are not only the building blocks of protein of which muscles and many other of the body’s parts are made, but also the basic constituent of DNA, which carries the genetic code for all living things.

Nitrogen atoms are also present in the molecules which enable energy transfer during photosynthesis. Without nitrogen, life as we know it would not exist.

Though about 78% of the Earth’s atmosphere is nitrogen, plants and animals don’t necessarily have an easy time getting all the nitrogen they need. Green plants can’t use the nitrogen that’s free in the atmosphere. Nitrogen must be “fixed” before it is usable by most living things.


The process of chemically altering unusable, free atmospheric nitrogen into a form usable by organisms is referred to as nitrogen fixation. In nature, there are two main ways of “fixing” nitrogen:

FIRST WAY: Lightning. If you’ve ever been close to a lightning flash and right afterwards smelled an ammonia-like odor, that was lightning-fixed nitrogen you smelled. Only a relatively small percentage of nitrogen gets fixed in this way, however. Nature’s main nitrogen fixers are…

SECOND WAY: Special microorganisms living mostly in soil and water.

Nitrogen-fixing microorganisms, existing abundantly but practically invisibly nearly everywhere, include a few forms of bacteria, the blue-green algae, and some fungi. Some nitrogen-fixing bacteria live in nodules, or small, bag-like growths on the roots of certain plants, especially members of the Bean Family.


In many backyards, nodules can be seen on the fine, wiry roots of clover, a member of the Bean Family, and considered a weed by those who don’t know its importance.

The image below is a much-magnified section of the roots of the clover in the above photo. The brown, baglike things hanging on the larger roots are nitrogen-fixing nodules.


Typically, nitrogen-fixing microorganisms do not fix free atmospheric nitrogen to a usable form in one step. Usually one set of organisms converts free nitrogen(N2) to ammonia (NH3). This ammonia is accompanied by its ammonium ion (NH4+), which some plants can use. However, most flowering plants need nitrogen in yet another form, which microorganisms provide by converting the ammonia to usable nitrate (NO3-).

Already you see that various organisms must work together to accomplish this profoundly important job. However, it’s even more complex than what’s described above! The process of converting ammonia to nitrate, callednitrification, is usually accomplished by two different sets of bacteria working one after the other.


The point of all this is not to convince you that nitrogen is wonderful stuff, although it is. The point is that nature is composed of a huge number of interrelated parts, and nitrogen with all of its jobs is just one tiny, usually ignored part.

When we dump toxic chemicals (insecticides and oil pollution,for instance) into the Earth’s air, water, and soil, we are upsetting vital life-enabling processes by killing organisms that are profoundly important to the continuance of Life on Earth.

13 thoughts on “The Nitrogen Cycle

  1. This reminds me of my old chemistry and biology classes at school and colouring in the nitrogen cycle!

  2. I must have missed that day in school… 🙂
    I learned this in my Master Composting Course. I think it’s so fascinating, especially the lightning part!!

  3. Cassie says:

    Thank you for some great information! I am just learning about permaculture and how to replenish the soil. I love to let the clover grow in our yard. It is pretty, great for our bees and farm critters, great for the wildlife, and great for our soil.

  4. Helping the different bacteria establish themselves in our aquaponics system was one of our first challenges after we filled it with water. It took at least four to six weeks before they were making enough nitrate for the plants. Waiting on them to establish themselves was like watching water boil. Thanks for the very interesting post!

    • Your project is fascinating!! I’d love to learn more about it. I will read your archives!!
      I didn’t think anyone would be interested in the nitrogen cycle but me. I’ve been pleasantly surprised!! 🙂

  5. katiepede says:

    I love the nitrogen cycle, I remember doing it at school…in fact rhizobium happens to be one of my favourite words 😀 x

  6. Thank you for this post! 🙂

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