What does the pH of the soil suggest about how to manage the garden?
This Lesson Plan is part the Gardening Module of SustainEd Farms' virtual programming.
Background: Testing the pH of your Soil
Testing the pH of a soil can determine the flow and availability of nutrients for microbes and, therefore, plants too. Essentially, the amount of hydrogen ions present (pH) in the root zone affects how well plants can uptake nutrients. The plants themselves attempt to create a gradient (a relative difference in hydrogen ions) necessary for transferring these materials (basically, plants pump hydrogen ions into the soil, causing an imbalance of charge, which equilibrates through diffusion of nutrients into the plant). These mechanisms also influence how easily nutrients are leached (put out of proximity of plant roots) out of the environment. Most plants prefer a slightly acidic environment (5.5 - 7.0) because those conditions promote optimal microbial activity (breakdown of matter into constituent parts). Measuring the pH of the soil, then, can inform a farmer of how to manage their farm with regard to adding amendments for improving productivity. In this lesson, substances with known pH values (vinegar -- an acid, and baking soda -- a base) are used to react with soil samples. A more basic soil (pH > 7) will react with an acid (the vinegar), while a more acidic soil (pH < 7) will react with a base (the baking soda).
Learning Objectives
Students will be able to...
Approximate the pH of soil in a garden, farm, or backyard
Understand which amendments may help to correct and buffer (resist changes to) soil pH
Academic Vocabulary
pH /piːˈeɪt/ noun. - literally “parts hydrogen,” it is a scale used to measure how acidic or basic (alkaline) a solution is
Directions
1. Gather your materials. You will need the following supplies:
soil (about 1 cup) · vinegar (½ cup) · baking soda (½ cup) · water · two bowls for holding contents · measuring cups and spoons
2. Watch the following YouTube video to see a demonstration of how to estimate soil pH. Then, follow along with the remainder of the lesson by reading the steps below.
3. Take a representative sample of soil from your garden, about 6 inches in depth. Take about a cup of soil to use. Spoon out about two tablespoons of soil into each bowl.
4. Check for basicity (alkaline soil): add ½ cup of vinegar to one of the bowls. If the solution fizzes within a few seconds, the soil has reacted with the vinegar, and this means that the soil is basic. If this occurs, then adding an amendment like sulfur to the soil will increase the availability of hydrogen ions (forcing them off of soil colloids). This will help the gardener achieve a slightly acidic soil, as is desired by most plants.
5. Check for acidity (acidic soil): add water to the soil in the other bowl until the mixture is muddy, then add ½ cup baking soda. If the solution fizzes within a few seconds, the soil has reacted with the baking soda, and this means that the soil is acidic. You may recall from the background information that plants prefer a slightly acidic soil. If the reaction seemed vigorous, it is possible that the soil may be too acidic. In this case, it is advisable to treat the soil with lime, which will help remove free hydrogen ions from the soil to increase the pH to an optimal level.
👀 Watch closely … for a chemical reaction to occur. A chemical reaction infers that one substance has gone a chemical change (rearrangement of molecular structure of original constituents), which can be noted by a change in temperature, an odor produced, a change in color, the formation of a solid (a precipitate), or -- for this experiment -- production of a gas (bubbles!).
6. If neither of the soil solutions fizz, then the soil has a relatively neutral pH (around 7). Adding sulfur to the soil may still be advisable in this case to achieve a slightly acidic pH. In any case, it should be noted that adding organic matter to soil -- along with the other amendments -- will help prevent large swings of pH that are characteristic of fragile or mismanaged agroecosystems.
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