Target Grade Level / Age Range:

Grades 9-12


50 minutes


  • Students understand that nitrogen cycles indefinitely through the Earth system
  • Students understand the places that nitrogen is found on Earth
  • Students understand that nitrogen is essential for life
  • Students learn that the cycle is nonlinear traveling between living things and the physical environment.


Suggested Companion Resources (books and websites)

Vocabulary (with definitions)

  • Nitrogen fixation: the process by which nitrogen gas (N 2) is changed into nitrates that plants can absorb through their roots
  • Ammonification: the process by which when plants and animals die, decomposers break down their remains and release nitrogen in the form of ammonium ions
  • Nitrification: the process by which nitrifying bacteria change ammonium ions into nitrites and nitrates
  • Denitrification: the process by which denitrifying bacteria convert some of the nitrates in soil back into nitrogen gas
  • Biological nitrogen fixing: when bacteria convert gaseous atmospheric nitrogen into nitrates
  • Atmospheric nitrogen fixing: when lightening charges, atmospheric nitrogen breaking the chemical bond and creating nitrates
  • Industrial nitrogen fixing: The Haber-Bosch process uses very high pressures to combine hydrogen and nitrogen from the air to produce ammonia. This process produces more than 500 million tons of artificial fertilizer per year
  • Nitrates: Nitrogen molecules that are easily absorbed through plant roots and can be used to make organic compounds
  • Assimilation: living organisms take up nitrogen and form it into organic compounds like amino acids

Background – Agricultural Connections (what would a teacher need to know to be able to teach this content)

Nitrogen is the most common element in Earth's atmosphere, yet it is also one of the most limiting factors for growth in plants and animals. Most nitrogen exists as a gas. In this common molecule, two nitrogen atoms are bound by a strong triple bond that makes them all but completely unavailable to any other atom, ion, compound, or organism. Yet without sufficient levels of available nitrogen, organisms would be unable to create their structures or to perform vital functions. Nitrogen is a key building block in a number of important molecules, such as nucleic acids, amino acids, and proteins. Without it, life as we know it would be impossible. The nitrogen cycle moves nitrogen back and forth between the atmosphere and organisms. Bacteria change nitrogen gas from the atmosphere to nitrogen compounds that plants can absorb. Other bacteria change nitrogen compounds back to nitrogen gas, which re-enters the atmosphere.

Interest Approach or Motivator

Ask students if they’ve ever heard someone say “Drink milk to get the calcium you need.” Ask students if they take, or know anyone who takes, vitamins or supplements. Have students explain why we need calcium or vitamins. Vitamins and minerals are full of the nutrients we need to be healthy – to build healthy bodies. Calcium is just one of those nutrients. Plants and animals need nutrients also. Three nutrients that are really important to plants and animals are nitrogen, carbon, and phosphorus.


  1. Today, we’re going to get familiar with the nitrogen cycle to see how plants and animals use and transfer nitrogen. The atmosphere is 80% nitrogen: why do you think plants and animals can't use nitrogen as it is found in the atmosphere? Can you explain what is meant by nitrogen fixation? What is the role of bacteria in the nitrogen cycle? Why don't legumes need nitrogen-containing fertilizers? Why is nitrogen so important for living things?
  2. Break your class into groups of four.
  3. For each group, provide objects to represent different steps in the nitrogen cycle. You can supply different objects to each group if you wish.
  4. Have students brainstorm possible relationships between the objects you provided.
  5. After a few minutes, have each group report their thoughts back to the whole class.
  6. Write students' responses on the board and have the class discuss which of the possible relationships seems the most plausible.
  7. For example, with the objects suggested students might suggest that the blown-up balloon with N 2 written on the side represents nitrogen in the air. The jar of fertilizer represents NPK soil amendments added to maintain healthy plants. Mushrooms would represent decomposers in the environment. Peanuts would represent legume plants that have a symbiotic relationship with bacteria to fix nitrogen. The photograph of a lightning bolt represents nitrogen being fixed. The toy animal represents eating plants to gain nitrogen in amino acids.
  8. Students won’t likely come up with all of the right answers. You can categorize their answers on the board into three columns of a KWL chart. Anything they know will go in the K column. Anything they want to know goes under the W column.
Objective 1
  1. Present the information on the Nitrogen Cycle in the accompanying PowerPoint slide show (slides 2-4). Have students capture notes in their notebook.
    1. Let's start with the air you are breathing. When nitrogen is in the air it is called atmospheric nitrogen and comes in the form of N 2, which means two nitrogen atoms stuck together. Plants can't do a whole lot with atmospheric nitrogen, but microorganisms like nitrogen-fixers, which are special bacteria that can change the nitrogen into a useable form through a process called nitrogen fixation. Let's take a look at how nitrogen fixation takes place.
      1. Atmospheric nitrogen makes its way into the soil where nitrogen-fixing bacteria on the roots of some plants change it to ammonium (nitrogen attached to hydrogen atoms, NH 4+). There are also some free-living bacteria (not on the roots of plants) that are nitrogen-fixers.
      2. Lightning can also change atmospheric nitrogen into nitrogen oxides, another type of nitrogen attached to oxygen atoms. This makes up only a small percentage of nitrogen fixation.
    2. Bacteria and archaea (another type of microorganism) in the soil change the ammonium into nitrites (NO 2-) and then nitrates (NO 3-) through nitrification, which is when bacteria change ammonium into nitrates. Nitrates are nitrogen attached to oxygen atoms.
    3. Now that the atmospheric nitrogen has been changed into nitrates, let's see what happens next. Assimilation is when plants use the nitrogen for all sorts of things like building leaves or making DNA (deoxyribonucleic acid). Animals and other organisms eat the plants, and the nitrogen gets incorporated into those bodies as well.
    4. Eventually plants, animals, and other organisms die and decay, releasing nitrogen back into the soil. Bacteria and fungi (mushrooms, for example) help break down the dead organisms, and through ammonification, nitrogen is turned back into ammonium. The ammonium is turned back into nitrates by bacteria (you basically go back to step 2).
    5. Special bacteria can turn nitrates back into atmospheric nitrogen through a process called denitrification, which is how nitrogen in the soil is released into the atmosphere again. And, you are back at step 1!
    6. So how did the nitrogen atoms in your turkey sandwich get there? Nitrogen from the air was turned into nitrogen that plants could use, like the lettuce and tomato on your sandwich. One day a turkey ate some of the plants and the nitrogen was incorporated into its body. But before all that occurred, the same nitrogen atom was eaten by a cow and released as feces where it returned to the soil (only to land in your turkey sandwich a few cycles later)!
  2. To solidify the process, have students write a story about it. Either in small groups, or as an entire class, have students assign names and characteristics to the different elements of the story turning components into characters. For example nitrogen could be named Nitro and could be the hero of the story. His super power is being flexible and able to be shaped by other individuals or environmental forces. Describe how nitrogen acts like other elements in the periodic table of elements or is dissimilar from other elements of the periodic table.
    1. Groups of students can choose from classic story lines like human versus human, human versus nature, or human versus self. Or they can create their own story line.
  3. Have students as individuals or in small groups write a story. Depending on time constraints, students can tell their stories to one another or to the whole group. Or they can develop the story in written form with greater detail to be turned in.  Note: the purpose of this activity is a creative way to develop understanding of the material. Embedding content of the nitrogen cycle is more important than the mechanics of grammar and paragraph structure.
Objective 2
  1. Present the information on the sources of nitrogen that plants can use in the accompanying PowerPoint slide show (slides 5-7). Have students capture notes in their notebook. Watch the suggested videos as well.
    1. Atmospheric nitrogen fixing: watch video https://
      1. Lightning can also change atmospheric nitrogen into nitrogen oxides, another type of nitrogen attached to oxygen atoms. This makes up only a small percentage of nitrogen fixation.
    2. Biological nitrogen fixing: watch video https://
      1. Atmospheric nitrogen makes its way into the soil where nitrogen-fixing bacteria on the roots of some plants change it to ammonium (nitrogen attached to hydrogen atoms, NH 4+). There are also some free-living bacteria (not on the roots of plants) that are nitrogen-fixers.
      2. Legumes are the types of plants that host these bacteria.
      3. Legumes include crops like alfalfa, clover, peas, beans, lentils, lupine, soybeans, and peanuts
      4. Farmers will often rotate their crops between a crop that uses nitrogen like corn with a legume crop. Having sufficient nitrogen available in the soil can increase crop yield.
    3. Industrial nitrogen fixing: watch video
      1. The Haber-Bosch process is an application of the nitrogen cycle for the benefit of man, since its credited with the production of synthetic nitrogen fertilizer which is responsible for the feeding a third to half the present world population. In fact, about half the nitrogen in each of our bodies is there thanks to the Haber-Bosch Process.
      2. Three to five percent of the world's natural gas production is consumed in the Haber process (around 1–2% of the world's annual energy supply)
      3. The Haber-Bosch process and development of widely-used, affordable fertilizers led to increased food production and likely a global population boom. In 1900 the world's population was 1.6 billion people while today the population is over 7 billion.
  2. Scientific Literacy/Problem Based Inquiry 30 min
    1. Have students read a variety of text/news story/articles (may differentiate based on ability or give the same text to all students) related to nitrogen ground water contamination in their town and globally.
    2. Example Texts:
    3. Have students share different issues related to excess nitrogen with the whole class from their various readings.
  3. Question (think-pair-share or other): We have discussed issues related to nitrogen in groundwater. What are the other locations/reservoirs of nitrogen in an ecosystem? How are managed cropping systems with added nutrients different from natural ecosystems? How do plants in nature get nitrogen? What are the pros and cons to applied technology that has resulted in industrially produced nitrogen?

Essential Files (maps, charts, pictures, or documents)

Did you know? (Ag facts)

  • Half of the nitrogen in our bodies is there thanks to the Haber-Bosch Process
  • There are three types of nitrogen fixation: atmospheric, biological, and industrial

Extension Activities (how students can carry this beyond the classroom)


Author(s) (your name)

Will Fett

Organization Affiliation (your organization)

Iowa Agriculture Literacy Foundation

National Agriculture Literacy Outcomes

  • T1.9-12.h. Understand the natural cycles that govern the flow of nutrients as well as the way various nutrients (organic and inorganic) move through and affect farming and natural systems
  • T2.9-12.a. Compare and contrast the differences between nature’s plant and animal lifecycles with agricultural systems (e.g., producers manage the lifecycle of plants and animals to produce a product for consumption)
  • T4.9-12.d. Evaluate the benefits and concerns related to the application of technology to agricultural systems (e.g., biotechnology)
  • T4.9-12.c. Discuss population growth and the benefits and concerns related to science and technologies applied in agriculture to increase yields and maintain sustainability
  • T1.9-12.h. Understand the natural cycles that govern the flow of nutrients as well as the way various nutrients (organic and inorganic) move through and affect farming and natural systems

Iowa Core Standards

  • HS-PS1-1. Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms. (nitrogen cycle and conversion of nitrogen and nitrous oxide to ammonia)
  • HS-LS4-4. Construct an explanation based on evidence for how natural selection leads to adaptation of populations. (corn planted closer together, crop rotation using legumes to fix nitrogen)
  • HS-LS4-5. Evaluate the evidence supporting claims that changes in environmental conditions may result in: (1) increases in the number of individuals of some species, (2) the emergence of new species over time, and (3) the extinction of other species. (increased nutrients like fertilizers can lead to increased yields)
  • HS-ESS2-2. Analyze geoscience data to make the claim that one change to Earth’s surface can create feedbacks that cause changes to other Earth systems. (water cycle, nitrogen cycle)
  • HS-ESS3-2. Evaluate competing design solutions for developing, managing, and utilizing energy and mineral resources based on cost-benefit ratios. * (adding nitrogen vs. crop rotation)
  • HS-ESS3-6. Use a computational representation to illustrate the relationships among Earth systems and how those relationships are being modified due to human activity. (water cycle, nitrogen cycle)

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