What's Your pH?
In this lesson students will measure the pH of a soil sample and learn how pH affects the availability of nutrient uptake by plants. Students will determine if and how their soil pH should be modified through the application of soil amendments. Grades 9-12
For the class:
- White vinegar (or 0.5 M solution of hydrochloric acid)
- Drain cleaner containing potassium hydroxide (or 0.5 M solution of sodium hydroxide)
- pH meters
- pH testing kit based on barium sulfate
- pH test strips (1-12 range) and litmus paper for quick acid/ base indicators
- Distilled water
- Internet access for research (this part may be done at home for homework)
For each group:
- Plastic 100 ml beaker
- 3 paper cups
- Spoon and stir stick
- Waterproof markers
For each student:
amendment: any material added to soil to make it more productive such as fertilizer or compost
compost: a mixture made of decaying organic material used to fertilize plants and amend soils
pH: power of hydrogen; a measure of the alkalinity or acidity of a substance
Background Agricultural Connections
This lesson is one in a series of related lessons to introduce students to chemistry and environmental science concepts. Activities are modeled after real-life challenges that modern farmers face while producing our food, fiber, and fuel. Labs are inquiry based and promote critical thinking skills. Other related lessons include:
Farmers are interested in the pH of their soils for a number of reasons. pH plays an important role in:
- The availability of essential plant nutrients.
- The activity of microorganisms in the soil.
- The solubility of some phytotoxic elements. For example, aluminum, iron, and manganese have greater solubility at pH values below 5.5. Sodium levels may become excessive when pH is above 8.5.
Soil moisture and organic matter also affect a plant’s ability to take up nutrients from the soil. Before nutrients can be used by plants, they must be dissolved in the soil solution. Soil pH is an indication of the acidity or alkalinity of soil and is measured in pH units. The pH scale goes from 0 to 14 with pH 7 as the neutral point. Soil pH can be altered through the application of elemental sulfur or lime (calcium carbonate). Application of lime or elemental sulfur is not an instant fix for soil pH. Depending upon the soil properties, it may take months or even a year for measurable changes in pH to occur after an amendment has been added to the soil.
Acidic, neutral, or alkaline (basic) soil is determined by the concentration of hydroxyl ions OH- and hydrogen ions H+ in the soil. Neutral soil has an equal combination of both hydroxyl and hydrogen ions; alkaline soil has more hydroxyl ions; and acidic soil has more hydrogen ions. From pH 7 to 0, the soil is increasingly more acidic, and from pH 7 to 14, the soil is increasingly more alkaline. pH stands for “power of hydrogen.” Since the pH scale is logarithmic, one unit of change represents a tenfold change in the pH. This means that a soil pH of 6 has 10 times more hydrogen ions than a soil with a pH of 7, and a soil with a pH of 5 has 100 times more hydrogen ions than a soil with a pH of 7.
Biological processes such as nitrogen fixation and decomposition of dead plant matter is optimal when pH is close to neutral. The availability of nitrogen, phosphorus, and potassium is best when soil pH is between 6.5 and 7.0.
In the western U.S., acid soils may be found in areas with sandy soils, high rainfall, and high levels of organic material. The weathering of granite and sandstone rocks also results in acidic soils. Limestone may be added to raise the pH of acidic soils. It does this by removing hydrogen ions from the soil.
Alkaline soils may be a result of arid conditions, soils high in sodium or parent rock material that is rich in calcium carbonate (CaCO). Elemental sulfur may be added to alkaline soils to lower the pH. Elemental sulfur forms sulfuric acid (HSO) when it reacts with oxygen and water in the presence of soil microbes. This lowers the pH. In small gardens, the addition of peat moss is also an acceptable way to lower the soil pH.
Soil that has a high amount of clay or organic material has a higher buffering capacity to resist a change in pH when base or acid forming materials are added. Soil texture and buffering capacity should be considered when determining the amount of acid or base forming material needed to adjust the soil pH.
For more information, see Answers to Commonly Asked Questions.
- Ask students if they know what an agronomist does. Introduce them to this career field by explaining that agronomists help farmers prepare and maintain their soil to achieve the maximum plant growth. They are an expert in the science and technology of producing and using plants for food, fuel, fiber, and land reclamation. They work in areas of plant genetics, plant physiology, meteorology, and soil science. Explain that agronomists help farmers achieve the maximum production from their land. They know the specific needs of plants and find methods of making soil as productive and fertile as possible.
- Watch the Agricultural Careers video clip, Agronomist.
- Inform your students they will:
- Explore soil pH
- Learn how soil pH affects plant nutrient availability
- Research how soil amendments may alter pH
- Use online resources to identify crops that grow best in slightly acidic or alkaline soils
Explore and Explain
- Before the lesson, prepare three soil samples. Samples should be close to 5.0, 6.5, 8.0 pH. This will require a little trial and error. Mix a little bit of soil with a lot of sand to make a soil mixture with little buffering capacity (the more clay and organic matter in soil, the higher the resistance to change in pH). The sandy texture of the soil will reduce the soil’s resistance to changing pH when you add acid or base forming materials.
- White vinegar may be used to lower the pH of the soil sample. (You may also use 0.5 M solution of hydrochloric acid if you need a stronger acid to change the pH or if you don’t want students to guess that the soil sample is acidic based on the vinegar odor.)
- A drain cleaner may be used to raise the pH of the soil sample. (You may also use 0.5 M solution of sodium hydroxide if you need a stronger base to change the pH.) Note: Traditional agricultural methods of adding lime to raise the pH and adding elemental sulfur to lower the pH takes months to alter pH, so we use drain cleaner and vinegar in this lab to instantly adjust our soil samples to our desired pH.
- Use an electronic pH meter to periodically measure the pH of the soil as acid and base are added until the desired level of acidity or alkalinity is reached. Label each soil sample 1, 2, and 3. The soil pH should be known by you, but not by your students.
- Tell students that in this lab they will act as agronomists, testing soil pH and advising farmers on methods for amending the soil for maximum crop productivity.
- Ask students if they know what pH is. Ask students if they can predict whether some common household items are acidic or alkaline. Do a demonstration to show students the pH levels of items such as lemon juice (pH 2.3), orange juice (pH 3.5), vinegar (pH 4.3), milk (pH 6.4), dish soap (pH 10), saliva (pH 6-8), or soda (pH 2-3).
- Ask students for ideas on why it would be important for farmers to know the pH of their soil. Use lesson background information to discuss the definition and importance of soil pH.
- Tell students that they will be testing soil samples that were sent in from three farms. Demonstrate to the students how to use each of the three pH testing methods.
- Use a pH testing kit based on barium sulfate in powdered form, where a small sample of soil is mixed with distilled water which then changes color according to the acidity or alkalinity.
- Use pH paper. A small sample of soil is mixed with distilled water into which a strip of pH paper is inserted. Show students how to compare results to the pH paper color chart.
- Use an electronic pH meter, in which a rod is inserted into moistened soil and measures the concentration of hydrogen ions.
- Instruct students to complete the information on their lab reports. When finished, discuss class results and have groups share their recommendations for each farmer based on the soil pH test.
- Instead of preparing soil samples of varying pH values, have students collect soil samples from their home, neighborhood, or community (while being respectful of private property). Test the pH of the soil samples and investigate explanations for differences in pH.
- In addition to testing the soil pH of the samples that the students collect, also test the soil texture. This is an important factor in the amount of lime or elemental sulfur needed to change the soil pH. Here’s a guide to soil texture by feel: soils.usda.gov/education/resources/lessons/texture
- Write down key terms so students can see them and connect them to the spoken word. If appropriate, connect a visual to each term introduced.
- As a class, create a flow chart to illustrate the procedure for the lab. Address questions that come up during the illustration process and prior to starting the lab.
Have students collect soil samples from your school garden. Test the pH of the soil samples and research the type of soil amendments that could be added to correct the pH level if needed. As a class, come up with a detailed plan to amend the soil pH, purchase and apply the soil amendment, and monitor the pH for any changes over the next couple of months. Soil samples should be cored from the first six inches of soil.
After conducting these activities, review and summarize the following key concepts:
- Agronomy is the science of soil management and crop production.
- Agronomists as well as Soil Scientists use their knowledge of chemistry and pH to help farmers achieve maximum plant growth. This in turn helps provide a more plentiful food supply.
- Nutrients can be added, or amended in the soil.
This lesson was funded in 2011 by the California Department of Food and Agriculture’s (CDFA) Fertilizer Research and Education Program (FREP). Chemistry, Fertilizer, and the Environment was designed to reinforce chemistry and environmental science concepts while educating students about the relationships between food, plant nutrients, farmers and the environment.
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