Permeability and Porosity of Soil
Teacher’s Instructions
Brief Description of Activity
Students are casted as part time real estate agents and part time soil scientists in
investigating potential real estate for the location of a new sports team. Field
conditions are one of the top priorities, and the team wants to ensure that the grass
will grow well and there will be minimal drainage issues. Samples from several
sites have been taken and should be evaluated for proper soil conditions. Neither
site is satisfactory, as the samples drain either too little or too much, so students will
be encouraged to design the soil that will cover the new field. Soil that drains a little
while still retaining moisture is desired, and students will combine several soil
ingredients in order to achieve this ideal soil.
Learning Goals
 Learn about the different components of soil.
 Learn about porosity and permeability and how different sizes of soil
particles affect these things.
 Learn about the role of surface area in soil.
 Utilize a graph with two axes to measure soil drainage.
 Create their ideal soil to be tested for permeability and porosity.
 Understand the importance of organic matter in soil and what it is useful for.
Recommended Grades: 6-12
Estimated Time Required: 60 minutes
Key Concepts and Explanation of Terms:
Porosity: Percent of soil that is void of matter.
Permeability: How easily water can move through soil.
Surface Area: Area of soil grains that are exposed.
Organic matter: Decomposing remains of plant material.
Sand: Soil particles ranging from 2.00-0.02 mm in diameter.
Silt: Soil particles ranging from 0.02-0.002 mm in diameter.
Clay: Soil particles less than 0.002 in diameter.
Micelles: Minerals that form plate-like structures in clays, that increases the surface
area and allows the clays to hold more water.
What Happens and Why:
Depending on how large the size of the soil particles is, the porosity will
either be higher or lower. With more surface area and a correlated decrease in
porosity, there is more space for water and other charged particles to cling on to soil
so that water will not drain as easily. With less surface area, the space is diminished
and water will pass through more quickly. What is needed for ideal growth of grass
is a mixture of several types of particles, large and fine. That way, a soil can hold on
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to enough moisture and minerals to permit plant growth without drowning plants
in excess water. The addition of organic matter helps soils structurally whether or
not soil is too wet or too dry. It improves moisture and nutrient retention in too dry
soils and helps drain overly wet soils, while providing nutrients to the ground.
Materials Needed (per group):
Sand
Clay or silty soil (or both)
Any other soil samples from different locations to test for fun
Organic matter, such as chopped leaves or peat moss
1 150 ml beaker with 20 ml measuring lines
1 250 ml beaker
1 400 ml beaker
1/2 cup measuring cup
¼ cup measuring cup
A funnel
Stopwatch
Deionized water
4+ coffee filters (in case of breakage)
2 Foam or paper bowls
1 popsicle stick for mixing
8 wooden cubes for a surface area demonstration (can be omitted or done
with other materials for the demonstration)
Plastic sheeting to put under experiment (optional)
Safety Information
None
General Outline of Procedures:
To demonstrate how rocks broken down increase surface area, put the
wooden cubes into a block (a “large” particle) and have students note the surface
area. Then take the cubes apart (“small” particles) and show how there is more
surface area when soil particles are broken down.
Students should be provided with the worksheet, deionized water, one cup
each of sand and silt or clay, along with a funnel, beakers, stopwatch, and several
coffee filters. Demonstrate the proper way to set up the funnel with a coffee filter
and dirt. The dirt should be down down in the bottom of the filter, but not pressed
down too far, just enough to cover the funnel completely. Set the dirt down into the
funnel and run a little bit of water through it initially to ensure that there are no
leaks in the dirt and filter. Make sure to tell them not to lift the coffee paper out of
the funnel after water has started running through, or else water may not leach
through soil and instead bypass through the coffee paper. Students will then run
one trial of water filtering through sand and one of water filtering through the
silt/clay. They will graph the milliliters of water in the beaker over time for both
experiments and compare.
The sand will drain too quickly, and the silt/clay will not drain enough, so
students will then be challenged to try and improve their soil using organic matter.
The students will mix one quarter cup of peat moss into their sandy soil and one half
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cup of peat moss into their clay soil. They will then take a half-cup of this mix and
run the time trial again. The peat should help silt/clay drain more quickly and the
sand drain less.
Distribute beakers of sand, silt, and organic material. Now, have students
add varying portions of the materials to a one half cup measuring cup. Then, put
this soil on coffee filters and pour water through, graphing the drainage like they did
with the sand and clay/silt. They will then make their recommendations for soil for
the new stadium after tinkering with several trials.
Clean Up
Soil can be disposed of in woods or a compost pile.
Sources:
Hydrology: Lecture 4 Porosity, Permeability and Darcy’s Law
Hofstra University
Accessed online 1/10/14 at:
http://people.hofstra.edu/j_b_bennington/121notes/pdfs/Porosity_Perm_Darcy.pd
f
Forest Ecology, Fourth Edition
Burton Barnes, Donald Zak, Shirley Denton and Stephen Spurr. John Wiley and Sons,
Inc. 1998.
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Porosity and Permeability of Soil
Student Worksheet
Name_____________________________
Introduction
The NFL has decided to add an expansion team. The newly formed
Waterville Worms are looking for a home location on which to build a stadium. The
new head coach spent many years at Sodden University, a school notorious for its
poor playing field conditions and would like to ensure that this new professional
team does not suffer from a notoriously bad playing surface. Thus, he has hired you,
part time scientists and part time real estate agents, to locate and decide on a
suitable site for the Worms to contest their home games.
In order for ideal conditions to be met, the soil of the stadium needs to drain
some so that the grass doesn’t drown, but at the same time, it shouldn’t drain too
much, lest there be no available water for grass to utilize. Different size soil
particles have different effects with regards to allowing the soil to drain. The
amount of open space within a soil column will affect how well the soil drains; this is
called the Porosity. Soil particles are classified into three main groups (with a
fourth, which we will get to later). They are:
Sand: 2.0-0.02 mm diameter
Silt: 0.02-0.002 mm diameter
Clay: <0.002 mm diameter
Take a look at the demonstration comparing large size objects in a mock soil
column. Looking at the size of the “particles”, which column has the most open air
space? Based on your answer above, what can you deduce about the relationship
between the size of the individual soil particles and that soil’s porosity?
Initial Candidates
Two sites have been selected in the initial round of property searching. They
are: Delta Flat and Riverbend Field. Have two members of your group come up with
two coffee filters and obtain one half cup of moistened soil from the Delta and one
cup from the Riverbend buckets in the front of the room to bring back to your lab
station.
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Materials
In order to test the soil at a site, a couple of materials are needed to run the
tests. For your group, gather up:
one funnel,
four coffee filters,
three beakers (150 ml, 250 ml, 400 ml),
One ¼ cup measuring cup for peat moss,
One ½ cup measuring cup for measuring soil,
a stopwatch,
two foam bowls,
a popsicle stick for mixing,
a bottle of deionized water.
Evaluating a Soil
Our first test will be for the Permeability of the soil. The permeability is the
measure of the soil’s ability to permit water to flow through its pores or voids,
meaning how quickly water can pass through a column of soil. As a soil’s porosity
increases, so does its permeability; with more space to flow in, water can travel
through the column more quickly. Take a pinch of each of the soils, one at a time,
and feel each in your hand. If porosity is the amount of open space in soil, based off
of the size of the particles, infer which soil you think will also have the most
permeability? Circle your answer. Riverbend or Delta
The experimental setup will be as follows: Put a half cup of the Delta soil into
one coffee filter and set the coffee filter inside of the funnel, setting the funnel
temporarily inside of the 500 ml waste beaker to hold it. Measure out 100 ml of
water into the 250 ml beaker, be precise, remembering where to read the meniscus.
In order to compare between sites, we are going to pour water through the
soil and measure the rate at which it gathers in the beaker below. The quicker it
gathers in the beaker below, the more permeable the soil. Before we pour any water
through the Delta sample, appoint one group member to operate the stopwatch.
Appoint another group member to watch the water in the beaker underneath, and a
final group member to record the times on the t-chart below. Starting at 20
milliliters, record time at every 20 ml increment. These two pairings can be used as
x,y coordinates, and can then be graphed to visually compare the rate of drainage
between soils. For the initial sand trial, mark the coordinates on the provided graph
with a dot.
Now that everybody knows what they are doing, transfer the funnel into the
150 ml measuring beaker and gently pour the 100 ml of water into the soil. At the
moment all of the water is in the funnel, the stopwatch operator should start the
watch. As stated above, record the time at which the water accumulates another 20
ml. Allow the funnel to drain for up to five minutes, or 300 seconds. At the end of
five minutes, estimate the final volume of the accumulating beaker for the last data
point.
After the Delta trial is complete, repeat the process with the Riverbend soil,
taking the Delta soil out of the funnel and setting it aside for the moment. Write the
Porosity and Permeability of Soil, CALEB HARRIS, 1/29/14
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milliliters that drain over time in the provided t chart, and then mark them on the
provided graph with a dot.
Water (ml) vs. Time for the Delta Permeability Trial
ml
H2O
20
Seconds
40
60
T Chart
Water (ml) vs. Time for the Riverbend Permeability Trial
ml
H2O
20
Seconds
40
60
Porosity and Permeability of Soil, CALEB HARRIS, 1/29/14
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Improving the Soil
After testing the two soils, what are your observations? What problems will
each soil have for field conditions in the Waterville Worm’s stadium? Write your
answers below.
Riverbend:
Delta:
Neither sample is acceptable for the client. The Delta sample is pure sand,
permitting excessive permeability while the Riverbend sample is a silty clay,
permitting little permeability. Fortunately, there is a solution available to nearly
everybody-organic material. That is, decomposing remains of former living things,
mostly plants, like compost or mulch. Organic material helps soils at both extremes:
in sand, it helps retain more moisture, and in clay, it helps improve permeability.
Let’s check out its effect on our two soils. Put each of the tested samples into one of
the foam bowls and bring it up to the front. Scoop a quarter of a cup of peat moss
into each bowl, bring them back to your lab station, and stir them up using the
popsicle stick. Take a half cup of this mixture, and put it into a new coffee filter.
When this is accomplished, run the same trials again with the new mixture. Record
the coordinates in the tables below and then graph them on the same axes that you
graphed your first trial on. Mark the coordinates for the organic matter trial with an
x instead of a dot.
ml
H2O
20
40
60
Sand
Seconds
ml
H2O
20
Seconds
40
60
Clay
Porosity and Permeability of Soil, CALEB HARRIS, 1/29/14
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How much was the soil improved by this addition? Do you think that this new soil
will suffice for the new stadium?
What other benefits might adding organic material have for the soil?
Discussion
Soil is quite complicated. In order for plants to grow, the soil must hold both
an adequate, though not excessive, supply of water and nutrients to help the plant
grow. Sand’s surface area is quite small per gram of soil. This makes it more
porous, and nutrient poor, since there is so little space to hold either water or
nutrients. Sandy soils tend to be poor soils, the only plants that survive there have
adaptions to withstand dry and nutrient deficient conditions. If it made up the soil
for a stadium, grass would not be able to grow. Organic material helps by slowing
down the rate of water loss and improving the soil’s capacity to hold on to nutrients.
Clay, on the other hand, has a lot of surface area, many times higher than
sand. Its micelles, mineral structures on the clay surface, permit it to hold water and
nutrients quite tightly. This can be a problem, however, when too much water is
held, and the soil cannot dry out, limiting plant growth. Stadiums with this type of
soil would be muddy, and the grass would have shallow roots, since it takes a lot of
energy to send roots down through the hard-packed clay. Neither condition is ideal.
Organic material will help soil structure by decreasing the surface area so that more
water can drain, and will add nutrients as it decomposes.
Final Activity
The site selection committee dislikes both spots for obvious reasons. Given
your expertise in soil, they have asked you to design the perfect soil to lay down at a
site that will be excavated and the previously poor soil present removed.
Loam is considered the ideal soil. Its ratio is 40 percent sand to 40 percent
silt to 20 percent clay, use this as a reference point. Given the materials at the front
of the room, create what you think will be the ideal soil: draining well, but not
excessively and having some ability to hold onto nutrients. It should add up to one
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half cup total, like the previous experiments. Repeat the same trial that you did for
the sand and the clay, and graph your results below.
What is your ideal soil recipe?
Water (ml) vs. Time for Your Designed Soil
ml
H2O
20
Seconds
40
60
How does your ideal soil compare to the other tested soils?
What other applications can you think of for soil design?
What soils in your life do you think could use some improvement?
Porosity and Permeability of Soil, CALEB HARRIS, 1/29/14
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Sources
Soil Mechanics and Foundations, Lecture 4.3
University of Connecticut
Accessed online 1/14/2013 at:
http://www.engr.uconn.edu/~lanbo/CE240LectW043permeability1.pdf
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