Megan’s Rain and Drain:
A Lesson in Soil Permeability
Background
(Vocabulary words are italicized)
Plant life depends on a few basic resources: sunlight, carbon dioxide, water, and
mineral nutrients. One of the important controls for plant growth is soil. Soil is important
to plant life because it provides, among other things, stability, water, nutrients, and access
to other symbiotic organisms such as fungi and bacteria. How effective soil is at
providing these things for plant life depends on key factors like: pH, mineral/chemical
composition, climate, depth and volume, texture and permeability. The various
combinations of minerals and organic matter produce different soil types, which can be
anywhere from dense, impermeable clay to loose, gravelly sands. The following lab will
test differences in soil permeability and how that affects water flow through the soil
samples.
Permeability of soil is the ability of water to infiltrate between soil particles, and
also to penetrate the soil and move through various soil layers. Infiltration usually is in
result of rainfall, but in this lab it will be performed by each lab partner.
When water soaks into soil a small amount is retained, temporarily, and then
soaks downward with the pull of gravity. In the same way that gravity pulls all objects
toward the center of the earth, it pulls water molecules downward through soil. In sandy
soils this is the primary cause of water draining downward through soil to groundwater.
When soils are not saturated with water, then the pores also contain a mixture of gases,
including nitrogen, oxygen, and carbon dioxide. Soil gases are produced and assimilated
by soil organisms, plant roots, and decay processes, and they are exchanged with gases
from the atmosphere. Without adequate exchange of gases in soil pores, crop growth
cannot occur because the oxygen needed by the plant roots would rapidly become
depleted. Also, if the soil becomes water-logged the pores can get filled with water and
the plant roots will suffer from the lack of oxygen and die.
Background information from: http://pmep.cce.cornell.edu/facts-slidesself/facts/wat-so-grw85.html
Science Topics: Scientific Method, Ecology, Gravity
Skills: Observing, Hypothesizing, Questioning, Measurement
Benchmarks:
1. Organisms: Recognize characteristics that are similar and different between
organisms
2. Heredity: Explain how related plants and animals have similar characteristics
3. Scientific Inquiry: Collecting and presenting data
4. Analyzing and Interpreting: Use the data collected from the investigation to
explain the results.
Time Required:

Prepping: 20 minutes

Developing: 10 minutes

Activity: 30 minutes

Cleanup: 15 minutes
Estimated Cost:

Soil $6.00

Sand $20.00 for 6 lbs

Paper Towels: $1

Rubber Bands: $2.00 for 900

Markers: $10.00 for 50

Plastic Cups: $4.00 for 60

Masking Tape: $ 1.00 per roll
Lab Objectives:

Compare water flow through soil and soil and sand mixture

Perform scientific method

Recognize differences in soil samples’ permeability and its affect on plant growth.

Recognize effects of soil saturation
Supplies Needed for 15 Pairs of Students:

4 plastic cups per pair (60 total)

2 rubber bands per pair (30 total)

1 cup soil per pair (15 cups total)

½ cup of sand per pair (7.5 cups total)

¼ cup water per pair (4 cups of water)

2 paper towels per pair (30 total)
Preparation: Gather all materials needed for the lab, discuss concepts, and before class,
instructor should prepare ¼ cup soil and ¼ cup sand mixture to be used to pass out with
soil for experiment. Also, have students make a prediction beforehand about whether they
think there will be any difference between the ways the water moves through the two soil
samples.
Lab Steps:
1. Using masking tape and a pen, label one empty cup soil and the other empty cup
soil and sand. Label other two empty cups water.
2. Attach a piece of tape on the side of each cup as shown in the diagram above. Use
a ruler to mark lines for 1, 2 and 3 cm measured from the bottom of the cup.
3. Next, use a paper towel to cover the top of the soil and the soil + sand cup, and
push the towels about halfway in. Fasten each paper towel in place with a rubber
band.
4. Place 1/2 cup of the soil sample into the paper towel in the soil cup.
5. Add the soil and sand mixture into the paper towel in the soil + sand cup.
6. Pour water up to the 2-cm mark in each water cup.
7. Add the water to the soil sample in each paper towel. Be sure to add the water to
each cup at the same time. Watch to see how much water drips into the bottom of
each cup. Record if there are any differences between how much water or how
fast the water drips into each cup? Measure the amount of water that drips into
each cup after 5 minutes. Compare how much water comes through each sample
with how much went in.
8. Now predict what will happen if you add the same amount of water again to each
sample, but give a reason for the prediction. Relate to the section concerning
saturation. Repeat step 7. What do you notice?
9. Make a bar graph representing how much water was added to each mixture and
compare to a bar graph representing how much water made it into the cup.
10. After the lab is completed 2 student pairs (4 students) could team up to grow
seeds in each of their strictly soil sample cups. One pair could empty the
remaining wet soil from their paper towel into the plastic cup and continue, over
days, watering the plant until the soil is saturated. The other student pair could
plant seeds in a plastic cup with drainage holes in the bottom, and continue to
water the plant in the same way as the other pair. The drainage holes will prevent
saturation and also prevent plant suffocation. The measurements of the plant
growth, or lack of growth, could be taken over a couple of weeks, and recorded.
The smaller the particles, the more water the soil can absorb and hold. If a soil has a lot
of clay, it can hold a lot of moisture. Sand allows water to drain through it more easily
than silt and clay so more water should have drained through the sandy soil than the
potting soil. The potting soil should have absorbed more water so less water should have
drained through it and into the cup. In step 8, when water was added again, about the
same amount should flow through the sandy soil and the potting soil. This would be true
if the potting soil absorbed all it could from the first wetting, and the excess water would
be able to drain through.
Extensions

Add vocabulary words to a weekly list in order to aid in comprehension

Create a bar graph representing the plant growth over a set number of weeks

Read books from the library concerning plants or soil characters, for example,
Diary of a Worm, or, Down Comes The Rain.
Transformations

Use different concentrations of sugar in water, and compare the sugar
absorption to explain saturation

Discuss the effect of gravity on a ball dropping from a rooftop, how does this
relate to water flow in a soil sample.
Assessment
1. Which type of soil allowed the most water flow, and in turn, is most
permeable? Why?
2. How does the amount of water retained in the soil affect the growth of a
plant?
3. What would happen to a plant’s root system if a flood occurred and did not
allow for drainage?
Vocabulary:
1. Symbiotic: an interdependent or mutually beneficial relationship.
2. Infiltration: the downward entry of water into soil.
3. Permeability: the ability to permit the flow of fluid through pore spaces
4. Soil: layers of matter formed by the decomposition of bedrock and organic
materials
5. Saturated: to soak thoroughly and completely