Lesson9 ~ -----~

Overview

Biological and agricultural concepts

Down and Dirty A study of water movement through soil

Soil is made up of sand, silt, clay, humus and living organisms. Soil texture, determined by what percentage of these materials is present in the soil, greatly impacts how water moves through soil.

The rate at which water moves through soil is an important factor determining if a soil can be used for growing crops, a building site, preserving an ecosystem or protecting the groundwater. This lab investigates the percolation rate of water through various soil types and demonstrates how to determine the water-holding capacity of soil.

The experiment is also designed to illustrate principles of experimental design, such as controls and independent and dependent variables, and the importance of repeating an experiment to verify results.

Water movement Water cycle Soil structure and drainage Experimental design Data collection and calculation Graphing

Percolation I 9-1

Lesson 9

The teachable moment

Background

9-2 I Percolation

Teacher material

Using simple bottle constructions, this laboratory can be used by both biology and agriculture teachers to teach basic soil science principles and to introduce students to Bottle Biology. In addition, it can be used to teach principles of experimental design, and data collection and manipulation.

Biology teachers can use this laboratory to teach introductory ecology and plant science. The agriculture teacher can use this laboratory to begin an in-depth unit on soils or plant nutrition, followed by labora­tories that relate soil properties to soil fertility management.

Soil is an integral part of the fabric of life. It nourishes plants, provides habitats for animals and microbes, and holds up our roads, cities, and homes.

A careful observer can see from roadcuts and rock outcroppings that soil does not look the same everywhere. You will find predominantly bright red clay soils in the Piedmont of Virginia, and deep, rich, black soils in the Midwest. Mountainous areas of the country have shallow soils overlying rock. On the coastal plain, you'll find pale, sandy soils. Differences in how the soils look mean differences in how soils func­tion in the environment.

Soil has been defined as "the collection of natural bodies on the earth's surface made of earthy materials, containing living matter and sup­porting or capable of supporting plants out-of-doors" (Harpstead 1988). The study of soil is called pedology. Today there are human­made soils and other planting mediums produced for gardening, hydroponics (growing plants in nutrient solutions), and tissue culture.

A study of soils is rich in applications to both science and agriculture courses. What is the source of nutrients for naturally occurring plants that are not fertilized? Why do some soils erode more rapidly than others, causing increased sediment loads in streams? Why does water pool in some places and soak readily into others? What are the impli­cations of these differences? How does soil type affect local ecology and vice versa? How do soils affect plant growth, microbial activity, chemical application, building construction, and other land uses? All of these questions probe at the fundamental importance of soil in our natural and human-made environments.

Soil is often considered "non-living," but if you take time to ponder it, you will find it has behavior and a personality that bring it to life. If

Lesson 9

9-4 I Percolation

Teacher material

such as limestone. Soils formed from stationary rock are called residual soils while those that form from translocated material are termed transported.

In addition to the physical effects of topography, climate and water movement, plants, animals and microbes actively participate in soil formation. Plant roots penetrate into small cracks in the rock, breaking apart the rock structure. The activity of insects and microbes further breaks down the rock.

Over time, soils form layers, or horizons, that are named for their characteristics. The topsoil, or A horizon, contains humus. The sub­soil, or B horizon, may be a leached horizon, where water has removed nutrients such as potassium and calcium. Clay often accumulates in this horizon. The C horizon is composed of highly weathered parent material, or very soft rock. This material may still be easy to spade. The R horizon contains the bedrock, the geologic formation that has given rise to the soil.

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Lesson 9 Teacher material

In the laboratory, soil scientists and others determine specific soil textures using particle size analysis and a textural triangle. Most soils can be informally categorized into one of three broad textural classes­clay, loam, and sand-based on the particle size that controls their behavior.

To read the textural triangle, locate a point on the chart. Read horizon­tally to the left and find the percent clay. The percent silt and sand are read on the diagonal lines that radiate from that point at 120 degree angles. '

100'• ClAY

Soil Triangle

Loam soils are composed of sand, silt and clay. Determining the tex­tural class of a soil sample collected from the local environment re­quires only the use of sight and touch. Try moistening a sample with a small amount of water and rubbing it together in your hand. A sandy soil will feel gritty, individual grains will be visible and the soil will fall apart when pressed into a ball. Clay soil will feel soft and creamy and will keep its shape when pressed into a ball. Individual grains will not be visible. A soil rich in silt will feel silky and the fine nature of the soil will darken and outline the fine ridges of your fingerprint. If a large amount of silt is present in a soil sample the soil most likely belongs to the loam textural class of soils.

Percolation I 9-5

Lesson 9

9-6 I Percolation

Teacher material

Water movement through soil is affected by many different factors. Gravity pulls water downward, while capillary action, or the attraction of water molecules to solid materials, tends to cause water to move more horizontally.

If you know something about a soil's texture (range of particle sizes), you can predict how it will behave in regard to percolation rate (how fast water will move through soil) and water holding capacity (field capacity). Soils made up of small particles provide more surface area for water to adhere to. Soil with small pore size holds more water and will drain more slowly. Soils composed of larger particles have less surface area, drain more quickly, and hold less water.

A mixture of particle sizes creates a mixture of pore spaces, so the percolation rate and field capacity of a soil will fall somewhere in be­tween the case of very small or very large particle sizes. Water flows downward through the pore spaces in response to gravity with a tendency for the water particles to stick together, minimizing total surface area (cohesion). At the same time, soil particles attract the water molecules to their surfaces (adhesion). As the amount of water becomes insufficient to cover the surface area and fill the pore spaces, water flow stops.

With the ever-increasing barrage of scientific information that students encounter, it is important that students understand the importance of testing a scientific hypothesis and what constitutes a valid experiment. Did the experimenter hold constant everything but the independent variable? Was the sample size used in the experiment large enough to avoid getting freak data?

Problem-solving in any field requires the same type of logical thinking involved in good experimental design. Every good experiment has clearly identified independent and dependent variables, a control or a standard, a large sample size or repeated trials, and holds constant all factors not being tested.

This experiment is a basic introduction to soil texture and behavior, one designed to stimulate more questions than it answers. Upon completion of this experiment, students will be able to relate particle size and pore space to two moisture storage characteristics: percolation rate and field capacity. They will gain an understanding of the cohe­sive and adhesive forces of water and the role of those forces in soil systems. This understanding serves as the basis for students to further investigate more complex relationships in the environment. Students

Lesson 9

Teacher management Preparation

Activity time

Tips

Teacher material

can also refine their measurement, data collection, and graphing skills using this activity.

For this experiment, you will need at least two different types of soil. Results from a local soil survey, available through your local extension office or Soil Conservation Service office, will be useful in selecting soil samples. Virtually every county in the U.S. has a completed soil sur­vey map available for the asking. These reports map the soil types in local areas and describe how these soils will respond when used. A soil survey will explain the variability of soils from community to community and will describe the reasons for such variation.

You can collect soil locally. Air-dry the soil for three to four days depending on the moisture level of the samples. Do not oven dry the samples as this can alter the structure of clay soils. H you want more specific types of soil, samples can often be obtained from the local extension office.

Day 1: build, set up and saturate columns Day 2: Measure field capacity and determine percolation rate Day 3 (optional): Calculate and graph results

Cutting the bottles, marking the soil column and saturating the soils will take approximately one 45-minute class period. This session will include a segment of time to review the procedure for the next lab. During the second class, students will collect data and discuss it.

A control soil should be added to experiments by preparing an equal mix of samples A and B. This will be sample C. A standard for com­parison may be created by using a professional mix such as Pro-Mix B.

Crush the dry soil samples to destroy any remaining soil structure. Sand samples that are single grained rarely require this; however, you will need to process finer grained samples, like clays, in this manner.

H you want to work with a heavy clay sample, the experiment will probably take longer than two class periods to run. If that will cause scheduling problems, test your most finely-grained sample to deter­mine if the experiment can be completed in two class sessions. H the sample is too heavy, mix in sand to a 3:1 ratio (3 parts fine-grained to 1 part sand).

Percolation I 9-7

Lesson 9

9-8 I Percolation

Teacher material

Students may have difficulty understanding that smaller particles provide greater surface area. To illustrate this concept, you might want to try using marbles and dried peas. For example, six medium­size marbles may fit into a film can while it takes 35 peas to fill the can. The radius of the marble is 1/2 inch and the radius of an average pea is 1/8 inch. Calculate the surface area of each size sphere (Surface area= 4n(r2). Multiply the number of spheres in each can by the surface area. Compare the total surface areas.

Marking the soil column as a graduated cylinder allows for easy mea­surement of the soil sample and provides an additional opportunity for data collection. If time is limited, a standard graduated cylinder can be used to eliminate the task of marking the soil column.

Make sure that each column is constructed using two bottles of the same brand name since bottle size varies slightly.

If you are teaching a unit on soils, the number of soil types can be increased and the types more carefully determined.

Horticulture instructors may wish to demonstrate the properties of the components of professional soil mixes using this design.

If time is limited, each group can collect data on a single sample and the class data can be pooled.

Pour water from the graduated cylinder onto the soil samples slowly, covering the whole soil surface. Be careful not to pour too fast and disrupt the soil.

Remember to crush soil samples after air-drying.

You may want to have a 2-liter bottle of water for each group to use.

A heavy clay sample may seal up before all of the water passes through the soil. This is a natural phenomenon that makes these soils particularly well-suited to use for building ponds. Be aware that this may be a problem.

In calculating field capacity, it is necessary to convert ml of water/300 ml soil to ml of water/liter of soil. Do this by setting up the proportion:

ml of water = x ml of water 300 ml of soil 1liter of soil

Lesson 9

Materials

Key terms

Teacher material

For each group:

• six 1-liter bottles, labels removed (all the same brand if possible) • 1 graduated cylinder (directions for making one using a

Schweppes bottle are in Appendix A.) • Exacto knife or other cutting blade • grease pencils or felt-tip marking pens • scissors • nail poke • 3 small squares of paper towel (approx. 2 cm2)

• stopwatch or watch with second hand • 3 film cans • 500 mls each of 2 air-dried soil samples of different textures • 500 mls of a mixture of the above two soils • water

Adhesion (adhesive): the attraction between unlike forces, like that between soil particles and water molecules

Capillarity: the interaction of forces that pulls water up in small spaces

Clay: mineral matter of less than 0.002 mm in diameter

Cohesion (cohesive): the attraction between like forces, such as the attraction of water molecules to each other

Dependent variable: the factor that is measured in an experiment. It changes as a result of the independent variable

Field capacity: the total amount of water remaining in a freely drained soil after the excess has flowed into an underlying unsaturated area

Independent variable: the factor that is changed in an experiment

Pedology: the study of soils as a naturally occurring phenomena, taking into account their composition, distribution and formation

Percolation: the downward or lateral movement of water through soil expressed as a rate-- volume of water per unit of time

Pore space: the continuous and interconnecting spaces in soils

Sand: mineral or rock fragments that range in diameter from 2 mm to 0.05mm

Percolation I 9-9

Lesson 9

References

9-1 0/ Percolation

Teacher material

Silt: mineral particles that range in diameter from 0.02 mm to .002 mm

Soil: a mixture of mineral matter, living and dead organic matter, water and gases found at the earth's surface

Soil profile: an exposed column of earth revealing the layering soil materials

Soil structure: the shapes formed when soil particles are held together

Soil texture: the degree of coarseness or fineness of soil particles due to the degree of weathering and parent material

Water holding capacity: (see field capacity)

Courtney, F.M, S.T. Trudgill. The Soil: An introduction to soil study . London, England. 1984. ·

Fitzpatrick, E.A. An Introduction to Soil Science. Longman Scientific & Technical: Essex, England. 1986.

Harpstead, Milo, Francis Hole, William Bennett. Soil Science Simplified. Iowa State University Press. 1988.

Hay, R.K.M. Chemistry for Agriculture and Ecology. Blackwell Scientific Publications Ltd: Boston, Mass. 1981.

McRae, Stuart G. Practical Pedology: Studying Soils in the Field. Ellis Horwood Limited: New York. 1988.

Plaster, Edward J. Soil Science and Management. Delmar Publishers Inc.: Albany, N.Y. 1992.

Lesson 9

Introduction

Student material

Down and Dirty A study of water movement through soil

Have you ever taken a walk on a beach and felt the warm sand mas­sage your feet? Have you ever gone wading in a creek and had the mud ooze up between your toes? If so, you have already made some discoveries as a soil scientist.

Soil supports plant life, serves as a foundation for buildings and roads, provides habitat for microbial life, and acts as a natural filter in the environment. Soil is defined as "the collection of natural bodies on the earth's surface made of earthy materials, containing living matter and supporting or capable of supporting plants out-of-doors."

Soil is made up of particles of sand, silt and clay. These particles range in size from smaller than 0.002 mm to 2 mm. That means the big particles are 100 times larger than the small ones! The texture of a given soil has to do with what mixture of soil particles is present.

After a rainstorm, water that lands on the ground will move through the soil. The rate and way it moves, and the amount that moves through versus the amount that runs off or is absorbed, is controlled by many different factors. Gravity tends to pull water downward. Capillarity can cause water to move horizontally. Other forces can cause the water to be bound tightly to the soil particle, making it un­available to gravity or capillarity. The texture of a soil can affect move­ment due to all of these forces. This soil texture affects the rate, pattern and quantity of water movement through a soil.

If a substance that is dissolved in water drains onto the ground, it may move through the soil just as pure water would. In such a case, the liquid might move through the soil into the groundwater and contami­nate the groundwater. However, in most cases some of the contami­nant will become bound to the soil particles and will not seep into the groundwater. The amount that binds to the soil and the length of time it will remain bound depends on the soil, the contaminant and the environmental conditions.

Two aspects of water movement in soil will be examined. Percolation, the movement of water though the soil, and field capacity, the amount of water a soil will hold, will be measured for soils of various textures.

Percolation I 9-11

Lesson 9

Materials

Procedure

Student material

Which soil texture permits water to move through more quickly? Which would you rather grow crops on if the climate were very dry or very wet? Which soil would you like to have over the ground­water that serves as your drinking supply? Test them and see.

• six 1-liter bottles, labels removed (all the same brand) • Exacto knife, razor blade or other cutting blade • grease pencil or felt-tip marking pen • scissors • nail poke • 3 small squares of paper towel (approx. 2 cm2)

• stopwatch or watch with second hand • water • 500 grams of two different soils (soils must be dry) • 500 mls of a mixture of the two soils above • 3 film cans

Day 1 - Preparation

1. Mark three bottles at the base for cutting. Using an Exacto knife or razor blade cut a small slit in the bottle and complete the cut with scissors. These will serve as the soil columns.

2. Mark another three bottles just below the shoulder and cut. These will be the column reservoirs. Cut away the base as shown, so you can see inside the column more easily. Mark 50 ml increments on the reservoir.

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A 15()

CUT ___________ .,..~

100 100

9-12/ Percolation

Lesson 9 Student material

3. Using a hole punch, punch one hole in the bottle base, 2 to 4 em below the top. The hole allows air to escape from the reservoir when air is displaced by water entering the reservoir from the soil column above. (Be careful not to pour water out the hole later in the experiment!)

4. Use a nail poke to make several holes in the center of each of three caps. Try to make the holes identical in each cap. Place a film can over each cap. This will prevent water from flowing out when you are initially saturating the column with water.

5. Place a small piece of paper towel in the cap. This will keep the soil from coming out the holes, yet allow water to pass through.

6. Place 500 mls of soil A in Column A,500 mls of soil Bin column B and a mixture, 250 mls of A and 250 mls of soil B, in column C. Rate each sample according to its particle size. Assign the soil with the smallest particle size a value of 1 and give the one with the largest particles a value of 10. Assign any remaining samples appropriate values between 1 and 10. Record these values in the data table.

7. Pour 300 mls of water on each column.

8. Let columns sit for 10 minutes.

9. Remove the film can from the cap.

10. Cover each column to slow evaporation and let the column drain for 24 hours.

Day 2 - Part I - Determining field capacity

1. Measure the volume of leachate that has passed through the column. Be careful not to squeeze the soil column when removing it from the base.

Percolation I 9-13

Lesson 9

9-14/ Percolation

Student material

2. Subtract the volume of the leachate from the total volume added (300 ml). The difference is the field capacity of the soil. Record this on data sheet.

Part II - Determining percolation rate

1. Pour 300 mls of water on the fully saturated soil columns used above. Start the timer.

2. Record the time it takes for 50 m1 of water to drain into the bottom reservoir.

3. Record the time it takes for the next 50 mls (total of 100 mls) to move through the soil.

4. Continue recording the time at 50 ml intervals until most of the water has moved through the column.

5. In most cases, the third and fourth interval will be the most accurate measurement of.the percolation rate. Take the average of those two measured times and convert the units to mls of water /minute.

Lesson 9 Data Sheets

Down and Dirty - Part I Determining Field Capacity

Name: ________ _

Soil Particle Initial volume Final volume Field capacity

type rating ~Gadded H20 in reservoir (Volume of water retained)

Sample A

Sample B

SampleC

Graph the data: Prepare a line graph relating particle size to water-holding capacity.

Water­holding capacity (mls)

5

Particle size

10

Percolation I 9-15

Lesson 9 Data sheets

Down and Dirty - Part II Determining Percolation Rate

Name: ________ _

Drainage rate

Soil A Soil B Soil C #of min. #of min. #of min.

Time in interval Time in interval Time in interval

SOml

100ml

150ml

200ml

250m!

Calculate percolation rate in mls/min:

Soil A ___ _ SoilB ___ _ SoilC ___ _

Graph the data: Prepare a line graph relating particle size to percolation rate.

Percolation rate (mls/min)

9-16/ Percolation

5 Particle size

10

Lesson 9

Results and discussion

Extensions

Student material

1. What is the independent variable in Part I? In Part II?

2. What is the dependent variable in Part I? In Part II?

3. Why was a mixture of the two soil textures used?

4. List three constants in this experimental set up.

5. Which soil has the greater field capacity? Why?

6. Which soil has the greater percolation rate? Why?

7. Which soil sample has greater pore spaces? How does this relate to field capacity? To percolation rate?

8. Describe any trend you see in the relationship of particle size to percolation rate.

9. Under what environmental conditions might a large field capacity be useful? Where might a large field capacity be a problem?

10. A large quantity of a toxic chemical was spilled on the ground. There is concern that the chemical could contaminate the groundwater that is 150 feet below the surface of the ground. All of the local wells get their water from the groundwater. How would knowledge of field capacity and percolation rate help predict the effects of the spill on the groundwater?

1. Try freezing any of the soil columns and thawing it. Does this change the nature of the soil water properties? What physical differences in appearance can be observed?

2. Select samples from each of the horizons present in a given profile and compare them. Place the data on a graph. Plot percent of clay on the x-axis and depth of sample on the y-axis. This plotting of information is called a clay curve or bulge. The clay bulge demonstrates the translocation of clay particles through the soil system.

3. To measure the impact of biotic components on soil water movement, add living organisms and repeat the experiment collecting the same data. Address how the results are different and what the results may mean in terms of land use and capability.

Percolation/ 9-17

Lesson 9

9-18/ Percolation

4. To examine soil microbial life keep soil moist and examine the sample daily. What creatures make the sample home?

5. Model a lawn. Vegetation on the soil surface can greatly affect the rate at which water moves into and through soil. Try sowing grass in the column and then redo the original experiment. Compare the results of the two trials. Maintain the grass, mowing it with scissors periodically and leaving the cuttings on the surface to build up a layer of thatch. Repeat the experiment to see the effect of thatch on the soil water movement.

6. Model a soil profile. Layer soil in the column alternating small particle size samples with larger particle size samples. Start simple, possibly with sand in the bottom and a clay sample on top and vice versa. Rate the movement of water from the large pores to the small pores. Are there any particular behaviors that can be noted? Construct more complicated layering patterns, possibly modelling a soil type in the area to give some insight on how water moves through. Remember the structure of the soil has been removed, so the water may move differently in the earth; however, this will give students a point of reference.

7. Would compaction of the soil affect water movement?