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How do plants get nitrogen

How do plants get nitrogen

Introduction: (Initial Observation)

Although the earth’s atmosphere is 78% nitrogen, we still have to feed the plants with nitrogen based fertilizers. Without nitrogen in soil, plants can not grow. Plant’s and animal life depends on nitrogen. My question is: Isn’t all the nitrogen in the atmosphere enough for the plants? Is there any difference between the gaseous nitrogen and fertilizer nitrogen? This project is an opportunity to find out why despite so much nitrogen in the atmosphere, we still have to add nitrogen compounds to the soil for plant’s growth.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.  

Project advisor

Some possible answers that can be tested are:

Maybe plants can not absorb nitrogen through leaves and they do it only through their roots. And maybe that is why we add nitrogen based fertilizers to the soil.

Maybe nitrogen has to be in the form of certain compounds in order to be useable by plants and that is why gaseous nitrogen is not good for plants.

Information Gathering:

Find out about nitrogen and it’s role in plants. Read books, magazines or ask professionals who might know in order to learn about different nitrogen compounds that are suitable for plants. Keep track of where you got your information from.

Following are some helpful information:

Plants use nitrogen in large quantities. Nitrogen is responsible for the lush green color we like to see in our plants. It is vital to plant growth, necessary for the production of amino acids and proteins, metabolic enzymes and photosynthesis. There is usually not enough nitrogen in our soils to satisfy plant needs and nitrogen is very mobile in soils leaching easily so we need to supplement with nitrogen fertilizer.

Nitrogen makes chlorophyll and combine with carbohydrates to form proteins.

Nitrogen Fixation

Plants require a number of different mineral elements for healthy growth, elements which may already be available in the soil or may need to be added in the form of fertilizers. One of the most important nutrients, one that is frequently in short supply, is nitrogen. This may seem strange when the atmosphere contains about 80% nitrogen. Unfortunately this nitrogen is in a gaseous form that plants are unable to use. Plants must obtain nitrogen in certain forms of nitrogen compounds such as ammonia and these forms are much less abundant.

There are organisms that can ‘fix’ or convert gaseous nitrogen into ammonia and they are referred to as ‘nitrogen fixers’. They are able to make large amounts of the nitrogen in the air available to plants through this activity.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

What part of a plant absorbs Nitrogen?

What compounds of nitrogen are usable for plants?


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

My hypothesis is that only roots can absorb nitrogen, however nitrogen has to be in the form of a water soluble nitrogen compound in order to be absorbed. Gaseous nitrogen and non-water-soluble nitrogen compounds can not be absorbed by the plants root.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment 1:

In this experiment we will test to see which part of a plant (root, stem or leaf) absorbs nitrogen. For this test we will use one of the nitrogen compounds that is well known to be an effective fertilizer. This can be Ammonium nitrate, Urea or ammonia.


  • 4 identical green plants. You may buy the plants or grow some seeds.
  • One pound nitrogen compound (Ammonium nitrate, Urea or ammonia)
  • clean water
  • spray bottle or small pint brush.


Remove 3 of your plants from existing soil and wash the roots. To do that first submerge your plant in water and then wash away all existing soil. We do that in order to avoid any damage to the root hairs. The fourth plant will remain at it’s own original soil to be our control.

Place all three plants in glass bottles.

Mark the bottles with labels showing which part of plant will be receiving nitrogen. Labels can say “ROOT NITROGEN”, “STEM NITROGEN” and “LEAFS NITROGEN”.

Prepare a 0.2% solution of your ammonium compound by mixing 2 grams (one tea spoon) of your compound with 1 liter of water.

Fill up one bottle with a 0.2% solution of your nitrogen compound and two other bottles with pure water.

Two times a day use spray bottle or paint brush to apply some nitrogen compound to the leafs of one plant and the stem of another plant. Make sure that excess compound does not enter the bottle. Also water the control plant and refill the bottles with water as needed.

Make daily observations, take pictures and record your results in a table like this:

Days CONTROL Root Nitrogen Leaves Nitrogen Stem Nitrogen

Your observation may include measuring and recording plant height, number of leaves, area of leaves, leaf color and general health of the plant.

Make sure all of your plants get the same light and are in the same temperature.

Experiment 2:

In this experiment we will test different nitrogen compounds to see which one will help plant growth. There are thousands of nitrogen compounds in the world but not all of them are accessible to us for our experimenting. However certain nitrogen compounds are accessible easily. A well known nitrogen compound that you can purchase from supermarkets is ammonia. Ammonia is a gaseous compound of Nitrogen and Hydrogen, however the ammonia that we buy from supermarkets is a solution of ammonia gas in water.

Two other nitrogen compounds that can easily be obtained are Ammonium nitrate and Urea. These two are both fertilizers and can be purchased from hardware stores and plant stores (nurseries).

Finally all proteins contain nitrogen. All of our body cells are made of proteins. Muscles (meat), bone, hair, nail, wool, leather are all different types of proteins and contain nitrogen.

Material and equipment:

  • 5 or more clean flower pots
  • 5 or more identical young plants that you may purchase or grow from seeds. Growing beans in a good quality potting soil is a good way of preparing your own test plants. Dig a one inch hole in the soil, place a bean and then cover it with soil. Water it once a day to keep the soil moist. In good weather conditions and light your young plants will be ready for your experiments in about 2 weeks.
  • proper lighting and temperature for plant growth.
  • 5 or more glass or plastic cups.
  • 5 or more nitrogen compounds. Try to have at least one water soluble nitrogen compound (such as ammonia, ammonium nitrate or urea) and at least one water insoluble compound such as hair, wool or grinded leather.


  1. Leave one of your plants as is in it’s original soil. This will be your control
  2. Carefully wash the rest of your plants and remove all the soil from the roots
  3. Fill up your plastic or glass cups with clean water. Label the cups with the name of nitrogen compounds that you want to test (urea, ammonia, wool, hair, …)
  4. Add each compound to the assigned cup as labeled.
  5. Place the plants in the cups in a way that the root stays in water and all other parts stay out.
  6. Use wires, strings, wood dowels or anything else that you can imagine to hold your plants in a normal or vertical position.
  7. Inspect your plants daily and record your observations in a table like this:
Day urea ammonia wool leather No nitrogen

Excess amount of nitrogen can seriously damage the plant. Many people who perform this experiment will notice the that plants with added nitrogen compounds will not grow and die fast. The reason is usually excess amount of nitrogen. If you use urea or ammonium nitrate I suggest to dissolve one tea spoon (2 grams) in 2 liters of water. If you use ammonia solution you can double this amount because ammonia solution already contains some water. So one tea spoon ammonia in one litter water should be sufficient.

Scientists who perform such experiments as a full time job, usually perform their experiments with large number of samples (not just 5). In this way they will have chance to test 10, 20 or even hundred different ratios of each nitrogen compound. In this way not only they will see the effect of certain compound, but they will also know what amount of that compound produces the best result. (You may also want to increase your number of samples).

If you test wool, leather, or hair as nitrogen compounds, you can use larger quantities of those. The reason is that those material are not water soluble and you will need large amounts of those in order to have a chance of seeing a result.

Research has shown that water insoluble nitrogen compounds have no effect on plant growth (you can test it yourself). However certain compounds like meat, will soon decompose (by bacteria) and change to water soluble nitrogen compounds. This will not easily happen with hair or wool or even leather, that is why we select them to test and demonstrate that not all nitrogen compounds will help plant growth. Another good thing that some bacteria can do is getting the gaseous nitrogen in air and convert them to water soluble nitrogen compounds such as ammonia. Many of these bacteria exist in the soil. We call them nitrogen fixers. Converting gaseous nitrogen to nitrogen compounds by bacteria is called nitrogen fixation. You can search the Internet for “Nitrogen Fixation” and you will find many material about that. One good thing is that many of these bacteria also live in the hair roots of some plants. Those plants are able to use the nitrogen of air (if root is exposed or somehow air can reach to the root).

Materials and Equipment:

List of material can be extracted from the experiment section.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


You will not need any calculation for this project.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.