California Agriculture in the Classroom

Plant Growth Affects the Soil (Grades 3-5)

Grade Level(s)

3 - 5

Estimated Time

Two 45-minute class periods plus optional activity

Purpose

Students will recognize nitrogen, phosphorus, and potassium as soil nutrients, learn that plants use soil nutrients as they grow, and that fertilizer replaces depleted nutrients. Students also analyze information on seed packets to learn more about the needs that different plants have for growth.

Materials

Activity 1:

Activity 2:

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

Essential Links

Vocabulary

organic fertilizer: a fertilizer that undergoes little or no processing and includes plant, animal, and/or mineral materials

fertilizer: substance used to increase the level of nutrients in soil

commercial fertilizer: commercially prepared mixtures of plant nutrients that include nitrogen, phosphorus, and potassium applied to the soil to restore fertility and increase crop yields. Commercial fertilizers contain nutrients in known amounts that plants can immediately use.

nutrient:  any of 17 essential mineral and non-mineral elements necessary for plant growth

nutrient deficiency: a condition where the amount of a nutrient essential to the health of an organism is lacking or present in an insufficient amount

Background - Agricultural Connections

 After completing this lesson, students will be able to:

See Preparation Instructions and thoroughly read each step of the "Procedures" section of this lesson for adequate background information.

This lesson is one of a 5 part series.  See the following titles for related lessons:

Interest Approach – Engagement

  1. Ask the students the following questions to introduce the lesson:
    • Do plants remove nutrients from the soil?
    • What replaces nutrients to the soil?
    • Do different kinds of plants need different environmental conditions for optimal growth.

Procedures

Activity 1: Plants, Soil, Nutrients, and Fertilizer

  1. Remind students of the word nutrients. Ask students if they remember what it means. If necessary, explain that nutrients are things that organisms need to be healthy.
  2. Write the terms “Vitamin C” and “Calcium” on the board. Ask students what they know about these things. After students express their ideas, summarize the idea that vitamin C and calcium are types of nutrients that people need to be healthy and we get these nutrients in the food and drink that we consume.
    • This comparison with humans should help students recognize that there are things (nutrients) in the food we eat that we can’t see but that are needed for health. Students may be able to give examples of the foods that contain these nutrients. For example, they may be familiar with oranges being a good source of vitamin C and milk being a good source for calcium and vitamin D. (If this is confusing to students, you might want to help them understand that the food contains the nutrients, but nutrients are not the same thing as food (food is important for providing energy). In later steps, students will be learning about nutrients in soil that they can’t see but are needed for plant health.
    • Note to Teachers: In Step 2, you have an opportunity to gauge students’ understanding that there are important things that are too small to see with our eyes alone and that organisms need certain things in order to live and be healthy. The difficulty with this analogy is that a common misconception is that plants “feed” (gain energy) off of nutrients in the soil. Plants use photosynthesis to create the sugars (energy) they need for plant growth and cell function. They take in nutrients from the soil to fulfill other functions. This is not completely analogous to how humans and animals eat food to get energy and to obtain nutrients.
  3. Write the following words on the board or chart paper: nitrogen, phosphorus, and potassium. Help students pronounce the words. Explain that students will be investigating these nutrients as they work through the rest of this activity.
    • It isn’t necessary at this point for students to have a deep understanding of these specific nutrients. Mainly, they need an introduction to substances as necessary nutrients for plants in preparation for the following steps of the activity. Remind students that they can’t see these nutrients in the soil, just like they can’t see vitamins in the food they eat.
  4. Display a transparency of Master 4.1, How Do Plants Affect the Soil?—An Example. Read through the information on the master with the class and use this as an example to teach students how to analyze this data.
    • After working through this example, teams of students will analyze similar information on other plants. The horizontal line marked “starting amount” represents the level of the nutrient before plants were grown in the soil. The bars indicate whether the amount of a nutrient in the soil either increased (more in the soil) or decreased (less in the soil) after the plants grew. If there isn’t a bar visible on the graph for a particular nutrient, the level in the soil didn’t change. (These numbers represent 50 pounds of crop harvest from an acre of soil.) Remember, the graph shows the change in the amount of these three nutrients after plants have grown in the soil compared with the amount before the plants grew. The graph does not show the specific amount of the nutrients in the soil.
    • Sample answers for the asparagus example on Master 4.1
      • There was LESS nitrogen in the soil after the plants grew.
      • There was LESS phosphorus in the soil after the plants grew.
      • There was THE SAME AMOUNT OF potassium in the soil after the plants grew.
    • Make sure students recognize that the example is from a specific type of plant or vegetable crop. Students do not need to be concerned with the specific numbers. Rather, the important thing for them to see is that there was less of a nutrient in the soil after the crop grew than there was before. Students also will observe that the amount of each nutrient removed from the soil was different.
    • Note to Teachers: Constructing and interpreting information from graphs is a skill that students need to practice. The graphs in this activity are likely to challenge your students. During the field test, working through the example (Master 4.1) helped students understand the big idea from the graph (and the subsequent graphs that they will look at). However, if you think your students would benefit from a visual example to learn how to interpret a graph such as this, you can use the following model with your students before working with the graph on Master 4.1.
      1. Draw a template for a bar graph on the board that is similar to the one below with the starting number of paper clips in the middle of the y-axis.
      2. Hold up a cup containing approximately 25 paper clips (the exact number is not important and this model is more similar to the data analysis of the crop plants if they do not how many clips are in the cup).
      3. Point out to students that the line labeled “starting amount” represents the amount of paper clips currently in the cup.
      4. While students are watching, add 3 more paper clips to the cup. Ask students how the number of paper clips in the cup changed. Students should respond that the number of paper clips increased (changed) by 3. You can explain that you are not asking for the total number of paper clips in the cup—just the number that it changed by.
      5. Draw a bar on the graph to represent the change (see A on the graph below).
      6. Remove the 3 paper clips that you added out of the cup. Help students recognize that the number of paper clips in the cup is the same as when you started. There would not be a bar visible on the graph (B on graph) at this point because the number in the cup matches the starting number.
      7. Now remove 5 paper clips from the cup and ask students how the number of paper clips in the cup has changed. Students should recognize that the number of paper clips has changed by 5 and there are now five fewer paper clips in the cup compared with the starting number. You can draw a bar on the graph to represent this change. (See C on the graph.)
      8. Continue the demonstration and add bars to the graph if helpful for your students.

  5. Ask  students to work with a partner. Give each pair of students one of the data sets on Master 4.2, How Do Plants Affect the Soil? Explain that they will look at data to see if there are differences in soil after plants grow.
    • As students work, circulate among teams to assess their progress, answer questions, or help teams that are struggling.
    • The basic format for each graph in Master 4.2 is similar. However, some of the y-axes have a different scale because of the data for the particular vegetable (an extreme example is the y-axis for the graph for peanuts). You might want to point out the scale as students are reporting their results. Including the larger values may make comparing the graphs slightly more challenging for students, but it is also an opportunity to point out things they should look at on graphs and to see how there are differences in plants and their effects on soil.
    • Note to Teachers: If you feel that your students are ready for this, ask each student to analyze one graph and then share their findings with their partner. In that way, each pair of students can compare results for two different vegetables.
  6. After students have had a chance to analyze their data, hold a class discussion to summarize the data. Ask teams to report which vegetable they analyzed and what conclusions they made from the data.
    • The main conclusion for students to draw from the data is that nutrients (nitrogen, phosphorus, and potassium) are removed from the soil when plants are grown. There is less of the nutrient in the soil after plants have grown than there was before (or no change for some nutrients). There are no examples given in which the nutrient level is higher after plants grow. There are differences among the plants in the amounts of the different nutrients removed, but this is less important for students to know than simply that nutrients are removed from the soil.
  7. After students conclude that the soil has fewer nutrients in it after plants grow, ask them to consider the following questions:
      • What happened to the nutrients that were removed from the soil?
      • Where do these nutrients go when the plants are harvested and taken away from the soil?
      • If there are fewer nutrients in the soil after plants grow, and the nutrients are taken away with the harvest, what do you predict will happen the next year when someone plants a new crop in that soil. Remember that plants need nutrients to grow and be healthy.
    • It is important that students understand that the soil contains less nutrients after plants grow because the plants took them up through their roots.
    • Taking nutrients out of the soil would lead to lower levels of nutrients in the soil. If the plants were allowed to decay in the same place, the nutrients they removed would be returned to the soil. However, when crop plants are taken away, the nutrients they removed from the soil are also taken away and the nutrients do not return to the soil. If the soil doesn’t have enough of the required nutrients, then plants would not grow as well and would not be very healthy. Even though the amount of nutrients removed from the soil seems very small, if plants are grown year after year, the soil can become depleted.
  8. Display a transparency of Master 4.3, Nutrients and Plant Health. Ask for volunteers to read information in the chart aloud to the class. Ask students if they can draw a conclusion about what happens to plants when they don’t get enough of a specific nutrient. Ask students if this supports their answers to the question in Step 7.
    • The main conclusion for students is that crop plants don’t grow well and aren’t healthy when they are missing nutrients. Students don’t need to know all the specific changes that occur in plants when nutrient levels are low.
  9. Continue the discussion by asking students if they think there is anything that can be done to put nutrients back into the soil.
    • Accept reasonable answers at this point.
  10. Explain to students that one of the things that can be done to replace the nutrients that come out of the soils is to use fertilizers. Fertilizers add nutrients back to the soil.
    • You may want to point out that the main nutrients in most fertilizers are nitrogen, phosphorus, and potassium—the same nutrients that students determined were removed from soil when plants grow. If students are interested, you can even point out that different kinds of fertilizers have different amounts of these different nutrients. For example, someone may choose a fertilizer that is higher in nitrogen and lower in phosphorus compared with a different fertilizer. In addition, fertilizers can either be organic (derived from decomposed, once-living things) or synthetic. In either case, the fertilizers contain similar fundamental elements.
    • Many students may think that fertilizer is plant “food.” This is not a scientifically accurate analogy, even though the term gets used this way in some popular media. A more appropriate analogy would be vitamins that humans take. People may take vitamins to replace nutrients that we don’t always get in our food.
    • Tip from the field test: One of the common misconceptions that students had was that fertilizers help keep soil moist. Be aware that your students may hold this misconception. If, upon probing, your students also have this misconception, ask them to explain why they think that this is the function of fertilizer. Help them understand that the purpose of fertilizer is to add nutrients back to the soil. It wasn’t clear during the field test what was leading students to this idea, but it could relate to the idea that nutrients are dissolved in water that the plants take up, that some fertilizers are in liquid form, or that the instructions on many fertilizers call for watering the plants after fertilizer is applied.
  11. Ask students to consider a scenario in which a farmer plants a crop in soil that is low in important nutrients. His plants aren’t doing very well so he puts fertilizer on the ground. Ask students to predict what effect this might have on the plants.
    • Students should expect that the crop plants would get healthier after the fertilizer is applied to the land. Plants don’t distinguish between the nutrients that are natural in the soil and the ones added as fertilizer.
    • Soils may be thought of as a “nutrient bank” that holds a limited amount of nutrients. Fertilizers put more “money” in the bank by restoring nutrient balance to farmed soils.
    • Students may ask whether you need to worry about using too much fertilizer. In fact, if the amount of a nutrient is too high, plants may not grow too well. For example, if a plant has too much nitrogen, it may not.
  12. Explain to students that farmers and gardeners often do soil tests to find out what nutrients are in the soil and what may be lacking. Ask students to think of reasons why a soil test can be helpful.
    • A soil test gives the farmer specific information about whether the soil has low levels or high levels of certain nutrients, such as nitrogen, potassium, and phosphorus (and other nutrients). This gives the farmer or gardener information that will help them make decisions about whether they need to use fertilizer, how much fertilizer to use, and even what kind of fertilizer to use. It can also help them make decisions about which crop plants may grow best in their soil.

Activity 2: What Should I Grow in My Garden?

  1. Ask students to list things that they would need to think about if they wanted to plant a garden outside.
    • Accept reasonable answers. If helpful, remind students what plants need to be healthy. For plants, the environment would include soil, water, and temperature, among other things.
  2. Explain that students will now be looking at the information on the packages that seeds come in to find out more about what different plants need to grow well in a garden. Ask students to work with a partner. Give each team one part of Master 4.4, Analyzing Seed Packet Information. Hold a class discussion in which students identify categories of information that they can find on the seed packets. Write the categories on the board or chart paper.
    • Pairs of students will have different information (information about different seeds), but most seed packets have similar types of information. (Depending on the number of students in your class, more than one pair may have the same seed information.) At this point, students don’t need to give information specific to their seed type, but rather focus on the type of information you can learn from the packets.
    • Some categories of information include:
      • Name of the seed or plant
      • A description of how the vegetable is used (prepared in dishes we eat)
      • How deep to plant the seeds
      • The type of soil that the plant grows best in
      • How far apart to space the seeds when planting
      • How far apart the plants should be*
      • How long it takes for the seeds to sprout
      • How tall the plants get
      • How long it takes until the plant is ready for harvest
      • The type of weather the plant needs (temperature, sunlight)
      • When to plant the seeds
    • Some seed packets may also give information about whether the plants grow well in containers.
    • *This may also be referred to as thinning. Usually, seeds are planted closer together than the plants should grow. This accommodates for seeds that do not germinate. On a seed packet, this may be referred to as plant space or thinning space.
    • Much of this information relates to the environment that is optimal for a specific type of seed or plant. This is another opportunity to reinforce understanding of the term, environment.
  3. Now ask teams to find two pieces of information on their handouts that are specific to their seeds. Ask them to share that information with the class in a discussion. Ask students why this information is important if you want to grow these plants in a garden.
    • For example, students with a lettuce seed packet may say that the seeds sprout in 7 to 14 days and that the plants should be spaced 8 inches apart. Students with a beet seed packet may say that beets grow well in early spring because they like cool soils and that you should plant seeds one inch deep.
    • Keep this discussion moving and brief. The purpose of this discussion is simply to demonstrate how different seeds and plants have different needs, and knowing this information about the different plants can help make a garden more successful.
  4. Conclude the activity by asking students if the information on the seed packets makes them think of other things they would need to know if they were going to plant these seeds in a garden. Record their responses on chart paper.
    • Students will likely give a variety of answers, including, they would need to know how large their garden is (how much space they have), whether it gets full sun, what type of soil is in their garden, what insects live in that area, or when the last frost of the season is in their location.
    • This is an opportunity to refer back to the list of things in a plant’s environment and things that plants need to be healthy that students generated in Lesson 1. Much of the information provided on the seed packets is directly related to the environment in which the plants will grow best.

Concept Elaboration and Evaluation

After conducting these activities, review and summarize the following key concepts:

Optional Activity: Fertilizer-How Much?

  1. Ask students to think about how much fertilizer should be used. Ask students to predict what might happen to plants if a very small amount of fertilizer was used or if a large amount of fertilizer was used.
    • Accept reasonable answers. If students understand the idea that fertilizers replace needed nutrients that were removed when a crop is harvested, then they might predict that too little fertilizer would not replace enough nutrients and the plants would still grow poorly. Students may have more difficulty predicting what might result from the use of large amounts of fertilizer.
  2. Ask students if they could design an experiment to investigate how the amount of fertilizer used affects plant growth.
    • Accept reasonable answers. If students understand the idea that fertilizers replace needed nutrients that were removed when a crop is harvested, then they might predict that too little fertilizer would not replace enough nutrients and the plants would still grow poorly. Students may have more difficulty predicting what might result from the use of large amounts of fertilizer.
    • The students’ ability to design an investigation will rely in part on their previous experiences with this type of task. One basic design is to use different amounts (concentration) of fertilizer. For example, if growing peas, some containers could be treated with fertilizer at the recommended amount (concentration if using a liquid), while other containers would be treated with either a more dilute solution or a more concentrated solution. Another set of containers would not receive any fertilizer as the control for the investigation. The containers that did not receive any fertilizer would serve as the experimental control. Other aspects of the experimental design to consider would be
      • Number of containers (plants) to receive each treatment (To be a good experiment, you will need more than one container at each fertilizer concentration.)
      • Location for the experiment (The plants/containers need to have the same temperature and amount of light as all of the others in the experiment.)
      • Soil and water (The plants should all be growing in the same type of container with the same amount and type of soil, as well as receiving the same amount of water.)
    • Students will also need to predict the results. An important aspect of experimental design includes having a clear prediction to test (hypothesis). For example, one prediction might be that plants that are fertilized with the recommended amount of fertilizer grow taller than plants that are not fertilized.
    • Note to Teachers: If you have fertilizer in your classroom, make sure it is labeled correctly, kept out of students’ reach and stored properly. Supervise students’ use of the fertilizer. Explain how the fertilizer should be handled.
  3. If time permits, students can work together to conduct this experiment and collect the data.

Enriching Activities

Suggested Companion Resources

Sources/Credits

Author(s)

Nutrients For Life Foundation

Organization Affiliation

Nutrients for Life Foundation