How Does Soil Affect the pH of Water?

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Abstract

Objective

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

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. Retrieved February 1, 2013, from http://staff.jccc.net/pdecell/chemistry/phscale.html
• United Stated Environmental Protection Agency (EPA). (n.d.). pH Scale. Acid Rain Students Site. Retrieved February 1, 2013, from http://www.epa.gov/acidrain/education/site_students/phscale.html
• United States Department of Agriculture. (n.d.). Agricultural Ideas for Science Fair Projects. Sci4Kids: Science, Agriculture, and You. Agricultural Research Service. Retrieved February 1, 2013, from http://www.ars.usda.gov/is/kids/fair/ideasframe.htm

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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:

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.

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

• pH
• Neutral pH
• Acidic
• Alkaline or basic
• Geosphere
• Biosphere
• Hydrosphere
• Runoff water
• Soil types
• Soil texture

Questions

• What value of pH is neutral? What ranges are acidic and basic?
• Can you think of examples of ways in which the geosphere, biosphere, and hydrosphere interact with each other?
• What is runoff water and why might it matter what the pH of that water is?
• How do scientists identify different soil types?

Bibliography

These are good sources for learning more about pH:

• Science Buddies Staff (2012). Acids, Bases, and the pH Scale. Retrieved September 6, 2012 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_AcidsBasespHScale.shtml
• Decelles, P (2002). The pH Scale. Retrieved September 6, 2012 from http://staff.jccc.net/pdecell/chemistry/phscale.html

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

• Bickelhaupt, Donald (2012). Soil pH: What it Means. Retrieved September 6, 2012 from http://www.esf.edu/PUBPROG/brochure/soilph/soilph.htm
• Pryzborski, Paul and Trakhtenberg, Izolda (2012, May 25). About Soil pH. Retrieved September 6, 2012 from http://soil.gsfc.nasa.gov/index.php?section=85
• BBC Staff (2012). How to be a Gardener: Analyzing Your Soil. Retrieved September 6, 2012 from http://www.bbc.co.uk/gardening/htbg/module1/soil_types1.shtml

Materials and Equipment

Supplies for this project are available in one convenient kit from the Science Buddies Store

To do this experiment you will need the following materials and equipment:

• Places to gather different types of soil (3)
• Small shovel or trowel for gathering soil samples
• Ruler, metric
• Sediment tubes with lids, 25 cm tall and 5 cm in diameter (9)
  • Small deli containers, like the kind sour cream or cottage cheese come in, also work.
• Permanent marker
• Small containers or cups, at least 3-oz. (3). They need not be identical.
• Tap water
• pH paper, with resolution of at least ±0.3 pH units and a range from 3 to 8 or a pH meter for liquids
  • You may need more than one set of pH paper to cover this range of pH values at ±0.3 pH unit resolution. See Table 2 in the Acids, Bases, & the pH Scale for more help.
  • Science Buddies Kit: pH 2.9–5.2 and another that goes from pH 5.5–8.0.
• Soil pH meter, with a pH range of at least 3.5 to 9 and a resolution of ±0.1 pH units
• Facial tissues or cotton balls (10)
• Clock, timer, or stopwatch
• Duct tape
• Coffee filters (18)
• Lab notebook

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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. Some pH papers are slightly different than others, but the general procedure is very similar. Take a look at the directions that came with your particular pH paper.
   2. Science Buddies Kit: Tear a 5 cm piece of pH paper from the roll. Dip the piece of pH paper in the water, and then compare the color of the pH paper with the colors on the case. 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.
   3. 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.
   1. Science Buddies Kit: 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. Stop adding water before it pools on the sample's surface.
      3. Shine the long narrow probe of the meter with the green pad but avoid rubbing 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,
         • 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.
         • If the pH is below 7, remove the probe, clean it with a new facial tissue or cotton ball, do not 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.
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?

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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?

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