How Does Soil Affect the pH of Water?

Objective

The objective of this experiment is to 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 the negative logarithm of the hydrogen ion concentration:

pH = −log [H⁺] .

What this equation means is for each 1-unit change in pH, the hydrogen ion concentration changes ten-fold. Pure water has a neutral pH of 7. pH values lower than 7 are acidic, and pH values higher than 7 are alkaline (basic). The table below has examples of substances with different pH values (Decelles, 2002; Environment Canada, 2002; EPA, date unknown).

Table 1. The pH Scale: Some Examples

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

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.

In this 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, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• pH,
• soil types

Questions

• What value of pH is neutral?
• What range of pH values is acidic?
• What range of pH values is basic?

Bibliography

• These sites explain the pH scale:
  • Environment Canada, 2002. "Kids' Corner pH Scale," The Green Lane: Acid Rain, Environment Canada website [accessed March 14, 2006] http://www.ec.gc.ca/acidrain/kids.html.
  • Decelles, P., 2002. "The pH Scale," Virtually Biology Course, Basic Chemistry Concepts, Johnson County Community College [accessed March 14, 2006] http://staff.jccc.net/pdecell/chemistry/phscale.html.
• NSSF, 1998. "Soil Quality Indicators: pH," National Soil Survey Center, USDA [accessed March 14, 2006] http://soils.usda.gov/sqi/files/indicate.pdf.
• Bickelhaupt, D. and R. Schmedicke, 2006. "Soil pH: What It Means," SUNY College of Environmental Science and Forestry [accessed March 14, 2006] http://www.esf.edu/PUBPROG/brochure/soilph/soilph.htm.
• These site have lots of information about soil, including soil formation, soil composition, soil types, and soil pH:
  • BBC, 2006. "Know Your Plot: Analysing Your Soil," How To Be a Gardener: The Complete Online Guide [accessed March 14, 2006] http://www.bbc.co.uk/gardening/htbg/module1/soil_types1.shtml.
  • Moor, F., 1999. "The World of Soil," [accessed March 14, 2006] http://web.ukonline.co.uk/fred.moor/soil/index.htm.

Materials and Equipment

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

• soil/water pH meter, with resolution of ±0.1 pH units, range 1 to 14 (available at nurseries/gardening stores);
• small shovel or trowel for gathering soil samples;
• plastic bags;
• marking pen;
• stick-on lables;
• places to gather different types of soil;
• plant pots (with drainage holes at bottom;
• paper cups;
• tap water.

Experimental Procedure

1. Do your background research so that you understand the terms, concepts, and questions, above.
2. Gather samples of different soil types. Here are some things to keep in mind:
   • For each different area or soil type, gather 10–15 small samples, and mix them together.
   • Keep each soil type in a separate plastic bag.
   • Label each of your bags.
   • Take notes in your lab notebook about the site where you collected each sample. 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.
   • Try to include as wide a variety of soil types and environments as you can.
   • For a method for determining soil type, see the Science Buddies project Get Down and Dirty: How Does Soil Change with Depth? You can use step 10 in the Experimental Procedure from that project to determine the soil texture, using a sample of soil from each site.
3. 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.
4. Use the soil pH meter to measure the pH of each soil sample.
5. Use the soil pH meter to measure the pH of the water before it contacts the soil.
6. Then, for each soil type, fill a plant pot about two-thirds full with soil. Try to use the same amount of soil (by weight) in each pot. (You can estimate the weight by hand, but if you have a scale that's even better.)
7. Add enough water to the soil in the pot so that it drips out the bottom, and catch the run-off in a clean paper cup.
8. Measure and record the pH of the run-off water.
9. Which soils change the pH of the water the most? The least?

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?

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

• Decelles, P., 2002. "The pH Scale," Virtually Biology Course, Basic Chemistry Concepts, Johnson County Community College [accessed March 14, 2006] http://staff.jccc.net/pdecell/chemistry/phscale.html.
• Environment Canada, 2002. "Kids' Corner pH Scale," The Green Lane: Acid Rain, Environment Canada website [accessed March 14, 2006] http://www.ec.gc.ca/acidrain/kids.html.
• EPA, date unknown. "pH Scale," What Is Acid Rain, U.S. Environmental Protection Agency [accessed March 14, 2006] http://www.epa.gov/acidrain/education/site_students/phscale.html.
• Staff, 2005. "Agricultural Ideas for Science Fair Projects," Agricultural Research Service, U.S. Department of Agriculture [accessed March 14, 2006]
  http://www.ars.usda.gov/is/kids/fair/ideasframe.htm.

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