The Importance of pH for Foods: An Investigation of Soils

The Importance of pH for Foods: An Investigation of Soils

Target Grade Level / Age Range:

6-8th grade

Time:

60 minutes

Purpose:

Students will be able to connect their knowledge of pH to real life agricultural applications, and understand the importance of pH requirements for Iowa crops.

Materials:

Suggested Companion Resources (books and websites)

Vocabulary (with definitions)

  • pH – a figure expressing the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are more acid, and higher values more alkaline.
  • alkaline – having the properties of an alkali, or containing alkali; having a pH greater than 7
  • acidic – having the properties of an acid, or containing acid; having a pH below 7
  • deficiency – not having enough of a nutrient
  • toxicity – having too much of a nutrient
  • stunting – decreased growth of a plant
  • chlorosis – yellowing of a plant
  • necrosis – the death of plant tissue

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

  • Acid/Base chemistry
    • Acids have a pH of 1-7, bases have a pH of 7-14
    • Acids produce H+ ions in solution. Bases produce OH- ions in solution.
    • Acids can be described as a proton or H+ donor, and bases can be described as a proton or H+ acceptor.
  • Effects of nutrient release from the soil based on pH levels
    • At the bottom of the Normal growth pH range sheet, there is a chart outlining the availabilities of different nutrients at a range of pHs. The main nutrients to note are nitrogen, phosphorus, and potassium. Those three are macronutrients. The rest are micronutrients, meaning the amount plants need of them are less.
    • For more information on macro and micronutrients, visit this page: https://soilsmatter.wordpress.com/2014/07/11/macronutrients-versus-micronutrients/
  • Basic knowledge of what plants types grow best in differing soil pHs
    • In Iowa, most of our crops thrive in comparable pH soils. However, some fruits and vegetables prefer more acidic soils. For example, blueberries are notorious for growing best in acidic soils (they prefer a pH between 5 and 5.5).
    • A related concept is one that as plants are harvested, bases are removed from the soil. Therefore, with each crop harvested, the soil can become more acidic. This isn’t necessarily a bad thing, but it makes it important for farmers to be aware of and manage their soil pH.
  • Managing soil pH
    • Over time and as a result of using nitrogen fertilizers, soils can become more acidic, so farmers need to control pH by “liming.” This term comes from limestone, which is the standard product that farmers may use to raise their soil’s pH.
    • There are many types of materials that can be used to raise soil pH, each with their own strengths and weaknesses. However, they aren’t all equal in terms of effectiveness, so there is an equation to help farmers and agronomists calculate the proper dosage of lime for a field.
      • Effective Calcium Carbonate Equivalent (ECCE) = Calcium Carbonate Equivalent (CCE) x Fineness Factor
        • CCE is determined in a lab, and is based on acid-neutralizing factor. Each material (dolomitic limestone, calcitic limestone, hydrated lime, marl, wood ash, etc.) will have this value assigned to it previously.
        • The fineness factor is calculated by measuring what percent of the material will pass through a 4-mesh, 8-mess, and 60-mesh screen (in Iowa – screen sizes are designated by state). The material is more efficient if it is more fine.
        • To see an example worksheet of how to use and understand this equation, visit this link: http://www.public.iastate.edu/~teloynac/354pplimsol.html
        • ECCE might not be necessary to explain in-depth to students, as it is pretty upper-level material, but exposing them to the idea that farmers need to use chemistry and math could be appropriate.
      • In other parts of the world where soils or crops require the pH to be lowered, things like sulfur, ferrous sulfate, or aluminum sulfate can be used.
    • Plant health and indicators
      • Some of the vocab words describe things that farmers and agronomists look for in their plants. Chlorosis and necrosis both relate to color of the plant. Chlorosis is when the plant turns yellow. Depending of the type of nutrient deficiency, the chlorosis could be interveinal (in between the veins), streaky, more circular, or a variety of other things. More information about chlorosis can be found here: http://extension.illinois.edu/focus/index.cfm?problem=chlorosis
      • Necrosis means plant tissue is dying or turning brown. This can also be a sign of nutrient deficiency.
      • It is important for farmers to pay attention to these indicators because at the end of the day, unhealthy plants are not profitable. They may not produce well, may not compete well with weeds, and can even become more susceptible to insects and disease. It is important to help plants stay strong and healthy so they have the ability to fight these exterior stressors. All of this can get out of whack if the soil pH is problematic.

Interest Approach or Motivator

  • Ask students the following questions:
    • What are some important things that you think soil does for plants?
    • Where do plants get their important nutrients from?
    • Why is it important for plants to get nutrients?

Procedures

  1. Explain to students that they are going to find the pH of different soil types. Have soil samples ready for the class before it starts, as well as all materials laid out in front of the students.
  2. Perform the pH soil analysis lab.
    1. Use large beakers to place soil samples in. If there are four different soil types, use four beakers. To make discussion easier, label them in some fashion.
    2. Mix the soil with water and stir.
    3. Siphon off water to put in a test tube. Have test tubes available for every student, every other student, or one per table. Have them pay attention to what soil type they are testing.
    4. Use either Litmus strips or a universal indicator for each test tube. Have students compare the colors to a pH color chart, and estimate their soil’s pH.
    5. A video of this process can be seen here: https://www.youtube.com/watch?v=1wpfA7yikOc
  3. Record students’ pH levels on the board at the front of the class, and average the samples from each soil type.
  4. Give the students the Normal growth pH range document (attached). Explain to them that each of the crops listed are grown in Iowa.
  5. Discuss with the class that pH levels of soil will determine what nutrients will be released from the soil surrounding the plants. Refer to chart at the bottom of the document.
    1. Touch on key nutritional topics:
      1. Difference between deficiency and toxicity
      2. Macronutrients (NPK) and micronutrients
    2. Ask students, “How can incorrect availability of nutrients harm plants?” and discuss.
  6. Have the students look over the list and explain to them that the list shows a list of food crops and the soil pH range at which each plant will grow the best. Ask students to mark plants that would grow well in the various soil samples from lab, based on their pH. Discuss the findings, patterns, etc.
  7. Finally, hand out the worksheet (attached document). Have them take a few minutes to answer the questions.
  8. End the class with a discussion. Summarize what they learned about soil chemistry and real-world applications of pH. Ask the students if they realized that farmers dealt with these issues. Ask for a few volunteers to share some take-away messages.
  9. Collect the worksheets at the end of class or at the beginning of the next class.

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

Did You Know? (Ag Facts)

  • Most plants grow between the pH range of 4.5 to 8.0
  • Most plants thrive in slightly acidic soil because that pH affords them good access to all nutrients
  • Too low of pH can render the plant nutrient manganese available at toxic levels. A pH that is too low also liberates aluminum – not a plant nutrient – in amounts that can stunt root growth and interfere with a plant’s uptake of nutrients.

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

  • Have students research how to raise or lower pH of soil. Then using the soil samples from this activity, have students use the ingredients they found from their research to lower or raise the pH of the soil to the required pH level for their specific plant.
    • Can follow up research with a math lesson on ECCE or percentages. 

Sources/Credits

Author(s) (your name)

  • James Sleep
  • Chrissy Rhodes

Organization Affiliation (your organization)

  • Bedford Community Schools – Middle School

National Agriculture Literacy Outcomes

  • T2.6-8.c- Identify farm practices for plant protection (e.g., using a pesticide, integrated pest management, cultural practices) and the harvest of safe products for consumers
  • T4.6-8.i- Provide examples of science and technology used in agricultural systems (e.g., GPS, artificial insemination, biotechnology, soil testing, ethanol production, etc.); explain how they meet our basic needs;

Iowa Core Standards

  • Science:
    • MS-PS1-2: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
    • MS-LS1-5: Construct a scientific explanation based on evidence for how environmental and genetic factors influence the growth of organisms.
    • MS-LS2-1: Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem.
    • MS-LS2-4: construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.
    • MS-ESS3-3. Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment.