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# How Does Porosity Vary in Rocks and Soils?

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### Talk It Over

Rocks may look solid, but air actually fills millions of tiny spaces between rock particles. Soils have air spaces between their particles, too. The porosity of a rock or soil is the part of its volume that is not occupied by solid material. Porosity is important because it affects how much water underground rocks can hold. It also affects how well or how poorly soils hold the water needed to support plant growth.

### Get

• Rocks of different types
• Oven
• Permanent marker
• Balance or kitchen scale (metric)
• Water
• Towel
• Soil samples
• Small baking dishes
• Measuring cups
• Cheesecloth

### Go

1. Place your rock samples on a cookie sheet. Ask an adult to put them in a warm oven (150° F) for several hours. This will get them completely dry. Let them cool completely before you go on to step 2.
2. Write a number or letter on each sample with a permanent marker so you won't get them mixed up.
3. Weigh each of the rock samples on the balance or kitchen scale. Record the weight in grams.

4. Soak the rock samples in water. Watch what happens. Make notes of your observations.
5. Let your rocks soak for several hours or overnight. Remove them from the water. Dry the rock's surface with a towel.
6. Weigh each rock on the kitchen scale. Calculate the grams of water that filled the pores in the rock:
wet weight – dry weight = grams of water in pores
7. Calculate the porosity of the rock:
grams of water in pores ÷ dry weight × 100 = percent porosity
For example, if a 100-gram dry rock weighed 130 grams when full of water, the amount of water in its pores was 30 grams. Its porosity was 30 ÷ 100 = 0.30 × 100 = 30%.

Note: Actually, porosity is defined by volume, not by weight. But since 1 gram of water occupies 1 milliliter of space, for convenience you'll use the two measures interchangeably.

8. Modify the procedure to test soil samples. Dry the soil in the oven in small baking dishes. Weigh the soil amounts. Flood with water and let sit for several hours. Pour the samples through cheesecloth to get excess water out and weigh again. Calculate the porosity of soils in the same way as you did for rocks.

### Stay Safe

Stay away from the hot oven. Let an adult dry your samples for you. Don't work with the rock and soil samples until they cool. If you collect soil samples, make sure you ask the owner's permission to dig.

### Go Easy

Test only rocks. Leave soil testing for next year.

### Go Far

Experts estimate the porosity of rocks and soils as follows:

Material Porosity (percent)
Soil 55
Gravel and sand 20–50
Clay 50–70
Sandstone 5–30
Limestone 10–30
Fractured igneous rocks 10–40

Accept these estimates as your hypotheses and conduct experiments to test them.

To learn more, find out about specific retention. In groundwater, it is the volume of water that will not flow through rock because it is held by surface tension on the rock's particles. The volume that can flow through freely is specific yield. It is always less than porosity. Find out how tightly packed and loosely packed rocks and soils differ in these properties; then design experiments to measure them.

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