Preparation

• 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.

Lab:

1. 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.
2. 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).
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.
4. 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.
5. 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.

Variations

• 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

ELL Adaptations

• 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.