To Fertilize Or Not to Fertilize

Grade Levels



Students will
  • Observe the growth and development of pea plants.
  • Use chemical and organic fertilizers on pea plants and compare the growth of each
  • Explore environmental issues dealing with the use of fertilizers.

Pa. Standards Addressed Doing the Activity

  • Teaching Method and Materials Assessment and Evaluation
  • Background Extensions
  • Getting Ready References

Pa. Environment & Ecology Standards Addressed:

  • 4.3.4. Environmental Health
    C. Understand that the elements of natural systems are interdependent.
  • 4.4.4. Agriculture and Society
    C. Know that food and fiber originate from plants and animals.
  • 4.6.4. Ecosystems and their Interactions
    A. Understand that living things are dependent on nonliving things in the environment for survival.

Other PA Standards Addressed:

  • Science & Technology
  • Mathematics
  • Reading, Writing, Speaking & Listening

Teaching Methods Incorporated

Hands-on, Inquiry, Observation,  Calculations, Data Collection, Experiment, Small group, Analyzing


  • Pea seeds (bush variety preferable)
  • Chemical fertilizer
  • Organic fertilizer (or compost – bought or homemade)
  • 3 garden locations that have the same exposure to the elements.
  • Handouts for each student
  • Pea Plant Data Sheet
  • Growth Comparison Chart
  • Ruler for each student
  • Pencil for each student
  • Journals for students (optional)


In order to grow, plants require essential minerals and elements, along with sunlight and water.  If the soil doesn’t provide enough nutrients for plants to grow, fertilizers may need to be added.   Fertilizers are like vitamins - they add nutrients to the soil.  But, like vitamins, they can’t replace a balanced diet.

A fertile soil has adequate amounts of the major plant nutrients – nitrogen, phosphorus and potassium; a sufficiency of the micronutrients (sometimes called trace minerals) – zinc, manganese, boron, iron, sulfur, an abundance of organic matter, and humus.  To be fertile, the soil must also have a nearly neutral soil pH as well as good structure and drainage.

Nitrogen, phosphorus and potassium can be obtained from natural sources (animal and vegetable matter) or from inorganic fertilizers that are chemically produced.

When you buy fertilizers commercially, most are labeled with a formula that states the percentage of nitrogen, phosphorus and potassium that the preparations contain.  The chemical symbols for these three elements are N, P and K respectively, and the elements are referred to by their symbols on fertilizer bags.  If a bag of fertilizer says it is “5-10-5,” for example, that means that it contains 5 percent N, 10 percent P and 5 percent K.  For example, a 100-pound bag of 5-10-5 would supply 5 pounds of nitrogen to your garden, 10 pounds of phosphorous and 5 pounds of potassium.  The rest is inert filler.  Both organic and inorganic commercial fertilizers are labeled using these initial letters.

Amounts of these elements can vary in fertilizer depending on what you want or need the plant to do.  Each element benefits a certain type of growth.  Nitrogen maintains plants’ green color and is largely responsible for good leaf and stem growth.  Nitrogen provides a quick boost of energy to a plant just like a candy bar does to us.  It is very soluble in water and thus leaches out of the soil quickly.  If your plants run out of nitrogen you will see pale foliage and spindly growth.

Phosphorous is especially important to the root development of a plant.  It also helps the plant to produce fruit and seeds and resist disease.  Root crops and flower bulbs require a lot of phosphorous.  It is not highly soluble and can be applied in the fall.

Potassium is essential for plant growth and for resistance to disease.  It is highly soluble and leaches out quickly so must be applied from time to time. 

Did you know that if you forget which element is P and which is K, you can use a memory device such as “Phosphorus” is P because the word has two P’s in it?

Fertilizers that have all of these elements are very popular and useful today, both chemical and organic, but they can also create environmental problems. Applying too much fertilizer means that runoff can be carried into surface waters and groundwater. Fertilizers enhance plant growth and therefore, can cause excessive growth in aquatic plants such as algae, and overwhelm the animals that eat it.  In addition, when the algae dies, the decomposing plant matter’s bacteria removes oxygen from the water and can suffocate fish and other plants. This can throw an entire ecosystem off balance. It is important to remember to follow directions on the label when applying fertilizer so it is not overused.

There are many reasons historically and practically for the development, promotion and use of chemical fertilizers. Mechanized transportation replaced the horse and, at the same time, many farmers stopped raising livestock. Something was needed to replace the manures as fertilizer. At about the same time, scientists became convinced that nitrogen, phosphorus and potassium were the principal plant nutrients and that by charging the soil with these elements in some usable form, crop yields could be increased and maintained at high levels year after year.

This activity gives students a chance to see how fertilizers work and whether organic or chemical fertilizers work better to encourage plant growth.

Getting Ready

  • Locate an area in the garden that can be used for this experiment.  You will need three plots – one for chemically fertilized plants, one for organically fertilized plants, and one as a control plot where nothing will be applied.  They should not be directly next to each other so that the fertilizers don’t leach through the soil to the plants in the other plots.  Even though plots are not next to each other, they should all have the same amount of exposure to the elements (sun, rain, etc.)
  • Buy an organic fertilizer and a chemical fertilizer with the same percentage of elements in them (i.e. 5N-10P-5K).  Note:  If you use compost instead of organic fertilizer, you will not have the percentage of elements but that is not detrimental to the experiment.
  • Photocopy the Pea Plant Data Sheet and Growth Comparison Chart – 1 for each student.

Doing the Activity

In the classroom

  • Ask the students what a “fertilizer” is and why gardeners use them.  Discuss organic vs. chemical fertilizers.  What are the differences?  What are the similarities?  Which do they think will work best on the plants in the garden?  If not used correctly, do they think fertilizers could be harmful to the environment?
  • Distribute a “Pea Plant Data Sheet” to each student.  Discuss the data sheet and how they will use it to collect data on the plants in the plots to which they are assigned.  Tell them that they will also be making written observations in their journals.

In the garden

  • Divide students into three groups – one group for each plot.
  • In mid-March have the students turn over the soil and plant pea seeds according to the directions on the package.  It is best to use low bush peas for this experiment.  If using vining pea plants, adequate support in the form of a trellis will be necessary.
  • After the pea plants have emerged, have students pick one plant to measure in their plot using a ruler and fill out the appropriate information on the data sheet.  Each student should use the same plant every time they collect data from this point on.
  • Have students apply fertilizer to the two “fertilizer” plots by following the directions on the packages.  The “control” plot will not have anything applied to it.  Reapply during the course of the experiment according to label directions.
  • Each week, preferably on the same day, have your student groups check their plots.  Using the data sheets and rulers, have your students measure their plants, fill out their data sheet for their plant and write down their observations in their journals.
  • After four weeks, have the students in each plot average the data of the plants in their plot. 
  • Distribute a “Growth Comparison Chart” to each student.  Have the students write the average height of the plants in each of the three plots on the chart.  Have them use different colors or three different types of markings to record each of the different plots of seedlings.
  • Students should repeat Steps 5 & 6 every week from now until the end of the experiment.
  • At the end of 10 or 12 weeks, look at the results.  Based on the Growth Chart, could the students tell which seedlings grew the most?  Was there a noticeable difference between the plots?  The results will vary so be prepared for any answer.  Explain to students that this means the fastest growing plants probably got most of the nutrients they needed.  In some cases, the plots without any additives will grow the best.  This indicates that the soil was already fairly nutritious before adding anything to it.  Soil like this may not need additional nutrients.
  • Finish the assignment by having the students write a report using the observations from their journals and their two data sheets.  Be sure they describe the purpose of the experiment, what they did and what they observed.  Have them conclude with a statement of what they learned.


  • Accuracy and completeness of Data Sheets
  • Evaluation of Journal entries
  • Evaluation of written report


  • Use a variety of different fertilizers and compost.
  • As a demonstration, collect rainwater in two cups.  Add fertilizer to one cup and leave the other cup alone.  You may observe algae forming quickly in the cup with fertilizer.  Discuss what happens when fertilizer runoff gets into surface water and how it can cause an ecosystem to become imbalanced.


  • National Wildlife Federation, Animal Tracks Activity Guide Grades 4-6, National Wildlife Federation and National Education Association Professional Library, 1995
  • Damrosch, Barbara, The Garden Primer, Workman Publishing Company, New York, 1988
  • Organic Gardening Magazine, The Encyclopedia of Organic Gardening, Rodale Press, 1978


Pea Plant Data Sheet