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How Does Soil Affect the pH of Water?

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Abstract

Did you know that soils can be alkaline, neutral, or acidic? Most plants grow best in soil near neutral pH, but some plants prefer slightly acidic and others slightly alkaline soil. What is the pH of the soil in your garden? What happens to the pH of water that comes in contact with soil? In this science project you will get to find out.

Summary

Areas of Science
Environmental Science
Geology
Difficulty
 
Time Required
Average (6-10 days)
Prerequisites
None
Material Availability
Readily available
Cost
Average ($50 - $100)
Safety
No issues
Credits

Terik Daly, Andrew Olson, Ph.D., and Teisha Rowland, Ph.D., Science Buddies

Sources

  • Decelles, P. (2002). The pH Scale. The Virtually Biology Course. Johnson County Community College.
  • United Stated Environmental Protection Agency (EPA). (n.d.). Acid Rain. Retrieved July 18, 2018.

Objective

Measure how contact with different types of soil changes the pH of water.

Introduction

The level of acidity or alkalinity of a soil is one indicator of the soil's health and suitability for growing particular types of plants. Acidity and alkalinity are measured with a logarithmic scale called pH. pH is defined as the negative logarithm of the hydrogen ion concentration, as shown in Equation 1 below.

Equation 1:

pH = −log [H+]

What does Equation 1 mean? It means that for each 1-unit increase in pH, the concentration of hydrogen ions (H+) decreases ten-fold. For example, something that has a pH of 6 has ten times as many hydrogen ions as something with a pH of 7, and 100 times as many hydrogen ions as something with a pH of 8, and so on. Pure water has a neutral pH of 7. pH values lower than 7 are acidic, and pH values higher than 7 are alkaline (basic). Table 1 below has examples of substances with different pH values. To learn more about pH you can also visit the Science Buddies webpage about Acids, Bases, & the pH Scale.

pH Value H+ Concentration
Relative to Pure Water		 Example
0 10 000 000 battery acid
1 1 000 000 sulfuric acid
2 100 000 lemon juice, vinegar
3 10 000 orange juice, soda
4 1 000 tomato juice, acid rain
5 100 black coffee, bananas
6 10 urine, milk
7 1 pure water
8 0.1 sea water, eggs
9 0.01 baking soda
10 0.001 Great Salt Lake, milk of magnesia
11 0.000 1 ammonia solution
12 0.000 01 soapy water
13 0.000 001 bleach, oven cleaner
14 0.000 000 1 liquid drain cleaner
Table 1. This table shows examples of substances with different pH values. (Decelles, 2002; Environment Canada, 2002; EPA, date unknown)

Soil, and all of Earth's other solid parts, together make up what is called the geosphere. As mentioned earlier, the pH of a soil affects what plants can grow there. This is just one way in which the geosphere interacts with the biosphere, which includes plants and all other life on Earth. Specifically, most plants prefer soil that is near neutral pH. There are particular varieties (strawberries, azaleas and rhododendrons, for example) that prefer acidic soil. Soil pH also influences how readily available many soil nutrients are to plants. The geosphere and biosphere are constantly interacting with each other, and with the hydrosphere, which includes all waters on Earth, such as in lakes, oceans, and the clouds.

In this geology science project, you will measure pH values of different types of soils, and you will see how the soil affects the pH of water that comes in contact with it.

Terms and Concepts

Questions

Bibliography

These are good sources for learning more about pH:

These resources can help you learn about soil types and soil pH:

Materials and Equipment

Experimental Procedure

  1. Do your background research so that you understand the terms, concepts, and questions, in the "Background" tab.
  2. Choose three different places to collect soil samples. You could choose places in your yard, a park, a garden, or other location with easily accessible soil. Make sure to ask permission before collecting samples, and be careful when collecting samples near bodies of water or roadways. Picking places with soils that are as different as possible will allow you to see whether all soil types behave the same or if soil pH and the pH of water runoff changes with soil type. Here are some suggestions:
    1. Look for soils with different colors and textures.
    2. Look for differences in the kinds of plants growing in a place. Different plants prefer different kinds of soils, so plant populations can be a clue to differences in soil type.
    3. Try sampling different environments, such as a floodplain, beach, man-made garden, forest, or desert.
  3. Take notes in your lab notebook about the sites. Include information about the general area (your yard, a park, the beach, a pine forest, etc.), and the kinds of plants (if any) growing in the area.
  4. Collect three samples from each location so that you can run the experiment for three different lengths of time. Remove the top 5 centimeters (cm) of the soil before collecting samples so you do not get any plants or surface roots in your samples. Also remove any stones or other objects from the sample
    1. Put each sample in a separate sediment tube. You will need around 2 to 3 cups of soil for each sample, depending on the size of your containers. Add soil to your containers until the soil is 15 to 20 cm deep.
    2. Label each tube using the permanent marker (e.g., "Soil from the riverbank, sample 1," "Soil from the riverbank, sample 2," etc.).
    3. Optional: Determine the soil texture for the soil from each location. Step 10 in the Procedure of Get Down and Dirty: How Does Soil Change with Depth? has a good method for determining soil texture. Record the soil texture for each location in your lab notebook.
  5. When you are ready to start testing your samples, fill one of the small containers or cups with about 4 cm of tap water. Use pH paper to measure the pH of the tap water, and record that value in your notebook. Then discard the water (use it to water a plant, etc.).
    1. Take a look at the directions that came with your particular pH paper. Some pH papers are slightly different than others, but the general procedure is very similar:
      1. Dip the piece of pH paper in the water, and then compare the color of the pH paper with the color scale on the packaging. Find the closest match to the color of the pH paper; the pH value associated with that color is the pH of your water sample.
    2. Note: You are investigating how the pH of water changes as it interacts with soil. To know how the soil changes the pH of water, you have to know the pH of the water before it mixes with the soil
  6. Read the instructions for your soil pH meter to learn how to use it properly. Make sure you follow any instructions for calibrating the pH meter before using it.
  7. Use the soil pH meter to measure the pH of each of the nine soil samples. Create a data table in your lab notebook to record the data.
  8. Clean the soil pH meter.
  9. Now you will investigate how the pH of water changes after it interacts with the soil samples for 1 hour. First, add more water to one of the sediment tubes from each site, until the soil is completely saturated and a layer of water about 1 cm deep forms above the surface of the soil.
  10. Cap each of the tubes, and securely seal the cap with a piece of duct tape. Mix each of the tubes thoroughly by covering the cap with your hand and then vigorously shaking the tube for one minute.
    1. Warning: Make sure the cap is securely sealed with duct tape and that your hand covers the cap as you shake the tube. The muddy mixture of soil and water will make a big mess if it escapes the sediment tube. So, shake the tubes vigorously, but cautiously. It may be best to do this step outside.
  11. Write the time on the sediment tube using a permanent marker. Make sure the outside of the tube is dry before writing on it. You will come back to these samples in one hour.
  12. Meanwhile, prepare three runoff-filtering containers.
    1. Put a coffee filter on top of each of the three, empty small containers or cups.
    2. For each container, fold the edges of the coffee filter over the edges of the container and tape the folded-over edges to the container with duct tape. Make sure to leave the top surface of the filter un-taped.
  13. After the sediment tubes have sat for one hour, remove the duct tape and cap slowly from the tubes. Slowly and carefully pour the water from the tubes onto the filters over the runoff-filtering containers. The water from each sediment tube should go into its own container, and you should keep track of which runoff samples are in which runoff-filtering containers. It is okay if some soil comes out while you are pouring off the water—that is why the filter is there! Let the water percolate though the filter.
  14. Once most of the water from the soil in the sediment tubes has been filtered, re-cap the sediment tubes.
  15. Carefully remove the filter paper from the runoff-filtering containers by removing the tape and holding the filter by its edges, being careful not to allow soil or unfiltered water to fall into the filtered water.
  16. Use pH paper to measure and record the pH of the filtered run-off water. Record the pH values in your lab notebook, adding the data to your data table and being sure to note which values correspond to which soil samples. Rinse out the containers when you are done.
  17. Calculate the difference in the pH of the tap water you measured in step 5 and the pH of each runoff you measured in step 16. This is how much the pH of the water changed after mixing with the soil for one hour. Record the difference in the data table in your lab notebook.
  18. Repeat steps 9–17, but let the soil and water mix for one day and then two days, instead of one hour. You will need to vigorously shake the sediment tubes every few hours to re-mix the soil and water.
  19. How did the pHs of different soil types compare to each other? Did any of the soils change the pH of the water? If so, by how much and how is that affected by the length of time the water mixes with the soil?

Troubleshooting

For troubleshooting tips, please read our FAQ: How Does Soil Affect the pH of Water?.

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Variations

  • For soils with pH that is more acidic or more basic than your tap water: if you keep watering the soil sample, does the pH of water eventually stop changing? Has the pH of the soil changed? How much water did it take? Does this vary with soil type? What does this tell you about irrigation and soil pH?
  • How does the addition of fertilizer affect soil pH? Does the fertilizer type matter? Do background research on fertilizers and pH and then devise an experiment to test fertilizer-induced pH changes.
  • Use an aquarium test kit to check nitrate levels in water drained from soil pots with and without fertilizer. Be sure to check a sample of the plain tap water too, as a control. Is there less nitrate run-off when plants are growing in the pots?

Frequently Asked Questions (FAQ)

If you are having trouble with this project, please read the FAQ below. You may find the answer to your question.
Q: How do I use my soil pH meter?
A: Soil pH meters come with instructions that you must read carefully and follow closely to collect accurate data. Every soil pH meter may be slightly different, so it is important to read and follow the directions. Here is an overview about how to use the RapiTest Digital Soil pH meter in the Science Buddies kit:

  1. Press the Power Button to turn the meter on and off. When you turn the meter on, its default reading is 7.
  2. To measure soil pH, slowly add tap water to the soil in the sediment tube until the soil is damp, but before water starts to pool on the sample's surface.
  3. Use the green pad that came with the meter to shine the long, narrow probe of the meter, but not the bullet-shaped tip. Use a facial tissue or cotton ball to wipe the probe from tip to handle.
  4. Gently put the probe in the soil in the center of the sediment tube.
  5. With the probe in the soil, twist the meter back-and-forth so that soil can attach to the probe. Twisting the probe is important to getting accurate measurements.
  6. Wait for 60 seconds. Then,
    1. If the pH is above 7, remove the probe, clean it with a new facial tissue or cotton ball, shine it with the green pad, re-insert the probe, and wait for 30 seconds. The measurement you read after 30 seconds is the pH of the soil. Record this in your lab notebook.
    2. If the pH is below 7, remove the probe, clean it with a new facial tissue or cotton ball, do not shine it, re-insert the probe, and wait for 30 seconds. The measurement you read after 30 seconds is the pH of the soil. Record this in your lab notebook.
Q: I am always getting a reading of 7 from my soil pH meter. What should I do?
A: If you are always getting a pH reading of 7 it is probably because you are not using the pH meter correctly. Re-read the instructions that came with the pH meter and make sure to follow them exactly. For the meter included in the Science Buddies kit, not twisting the meter and not waiting for the appropriate amount of time are the most common causes of a constant pH 7 reading.
Q: The filter in the runoff-filtering containers ripped, and unfiltered water or soil fell into the container. What should I do?
A: This is an easy problem to solve. Create a new runoff-filtering container and simply re-filter the water.

Ask an Expert

If you have other questions about the procedure or need assistance troubleshooting your project or the Experimental Procedure, please post your question in the forum for this kit at Ask an Expert: http://www.sciencebuddies.org/science-fair-projects/phpBB3/viewforum.php?f=74. Our team of volunteer Experts is available to assist. We attempt to reply to questions within 24 hours. Please note that you will need a free Ask an Expert account in order to post questions.

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MLA Style

Science Buddies Staff. "How Does Soil Affect the pH of Water?" Science Buddies, 13 Oct. 2022, https://www.sciencebuddies.org/science-fair-projects/project-ideas/EnvSci_p013/environmental-science/how-does-soil-affect-the-ph-of-water. Accessed 7 June 2023.

APA Style

Science Buddies Staff. (2022, October 13). How Does Soil Affect the pH of Water? Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/EnvSci_p013/environmental-science/how-does-soil-affect-the-ph-of-water


Last edit date: 2022-10-13

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