Retaining Water in Soil

Author

Brett Welsch

Published

1/4/2017

Topics:

Lesson Plans - 3rd - 5th

Target Grade Level / Age Range:

5 ^th Grade

Estimated Time:

50 minutes

Purpose:

Students will learn that different types of soil have various water-holding capacities and that it influences how well crops grow in that soil. Soil texture and structure is important for sustaining productive plant life.

Materials:

Small nail
4 foam cups, 12 or 16 oz.
Metric measuring cups
Dry clay
Sand
Silt
Potting soil
4 clear plastic cups, 9 oz.
Water
Stopwatch

Essential Files (maps, charts, pictures, or documents)

Vocabulary (with definitions)

Soil: A mixture of organic material, clay, and rock particles in which plants grow
Sand: Largest soil particle that does not hold water and nutrients well
Clay: Smallest soil particle when dry, has a smooth texture but when wet is sticky
Silt: Particles are in the medium sized. Dry feels powdery like flour but allows water and air to mix in the soil
Loam: Properties of all three kinds of particles to form an especially fertile soil
Soil Texture: A classification determined by the fraction of sand, silt, and clay presented in the soil.
Horizons: Soil profile that consist of multiple layers
Topsoil: The top layer of soil that plants germinate and grow in.
Subsoil: The soil layer immediately under the surface soil.
Bedrock: Bottom layer of soil profile that contains solid rock.

Background – Agricultural Connections (what would a teacher need to know to be able to teach this content)

Types of soil: There are three soil textures that make up a soil type. They are sand, silt, and clay. Each texture has different particle sizes which determine the pore space of the soil. Pore space is where roots can grow in the soil and it is also what hold water and air, which are two vital components of creating a soil profile and allowing a plant to grow.

Sand: Sandy soils drain very fast and do not hold water very well. They have the largest particle size and feels gritty to touch. Sand particles do not bind very well and easily warms up in the Spring because they have more pore spaces that fill with air. Its degree of aeration depends on the sizes of the particles, which vary a lot in size. Sand is usually formed from the weathering or disintegration of bedrock such as shale, limestone, granite and quartz.

Silt: Silty soils are finer, and smoother in texture and hold the most available water to plants. Sand does not hold any water and clay particles hold water so tightly to the particle surface that plant roots are unable to extract it from the soil. Silty soils are also heavier than sandy soils, and holds up nutrients and make it better for crop cultivation. Silt is formed when fine sediments (dust, organic matter, and debris) are carried by water or ice and deposited. When silt is deposited and cemented with time, it forms siltstone. Silt particles are very small and not easily seen by the eyes.

Clay: The particles that make up clay are the finest and they bind very well to each other and to water particles. Since they bind so well together they have very little air spaces. Clay is very sticky when it is wet and can be molded into any shape and form. When clay dries it is rock hard. Clay particles also do not drain water very well which can be a problem for plants. You do not want to underwater plants but you also do not want them to be submerged in water with no ventilation. Clay particles are so small that they are millions of clay particles that can fit in the size of your pinky fingernail.

Loam: Loam is considered the perfect soil. This soil is a combination of sand, silt and clay. This soil is gritty and retains water while also having good drainage. It normally has the highest amounts of organic matter. Loam is considered the best to grow most any type of plant.

Interest Approach – Engagement

Display or pass around four different soil samples (sand, silt, clay, and loam). Pose the following questions to students and capture some of their responses on a large writing surface.

Does the percentage of sand, silt, and clay determine the water holding capacity of soil?
Which of the four soil (sand, silt, clay, loam) samples do you think will hold the most water and why?
Which soil is best for growing corn and why?
How can a person mix clay, sand, and potting soil to obtain a soil sample that will retain 50% of the water it receives after 2 minutes?

Procedures

Present the following information to students. Have them capture notes, key terms, and other information in their notebooks.
1. Sand: Sandy soils drain very fast and do not hold water very well. They have the largest particle size and feels gritty to touch. Sand particles do not bind very well and easily warms up in the Spring because they have more pore spaces that fill with air. Its degree of aeration depends on the sizes of the particles, which vary a lot in size. Sand is usually formed from the weathering or disintegration of bedrock such as shale, limestone, granite and quartz.
2. Silt: Silty soils are finer, and smoother in texture and hold the most available water to plants. Sand does not hold any water and clay particles hold water so tightly to the particle surface that plant roots are unable to extract it from the soil. Silty soils are also heavier than sandy soils, and holds up nutrients and make it better for crop cultivation. Silt is formed when fine sediments (dust, organic matter, and debris) are carried by water or ice and deposited. When silt is deposited and cemented with time, it forms siltstone. Silt particles are very small and not easily seen by the eyes.
3. Clay: The particles that make up clay are the finest and they bind very well to each other and to water particles. Since they bind so well together they have very little air spaces. Clay is very sticky when it is wet and can be molded into any shape and form. When clay dries it is rock hard. Clay particles also do not drain water very well which can be a problem for plants. You do not want to underwater plants but you also do not want them to be submerged in water with no ventilation. Clay particles are so small that they are millions of clay particles that can fit in the size of your pinky fingernail.
4. Loam: Loam is considered the perfect soil. This soil is a combination of sand, silt and clay. This soil is gritty and retains water while also having good drainage. It normally has the highest amounts of organic matter. Loam is considered the best to grow most any type of plant.
Have students follow the following procedure for the lab. If time and supplies allow, you can have groups of students complete the experiments. Otherwise, do the experiment once as a class demonstration. Pass out a copy of the Soil Water Holding Capacity worksheet to each student.
1. Use the nail to punch 10 tiny holes in the bottom of each foam cup.
2. Measure 250 mL of clay. Put the clay in one of the cups.
3. Follow the same procedure with the sand, silt, and potting soil.
4. Place each foam cup in a smaller plastic cup. The water that seeps through the holes will collect in the plastic cup.
5. Pour 160 mL of water over the clay in the foam cup. Start timing when as the water is poured on the soil.
6. Let the water drain into the plastic cup for 2 minutes. (Ensure that a vacuum has not been created between the two cups. A vacuum will prevent the water from draining into the plastic cup.)
7. After 2 minutes, remove the foam cup from the plastic cup. Measure the water that drained into the plastic cup by pouring into the metric measuring cup and recording the reading.
8. Record your observations and measurements in the chart.
9. Repeat steps 5-8 with the sand, silt, and the potting soil.
Have students respond to the questions on the bottom of the worksheet. Review the answers to wrap-up the lab activity.

Did you know? (Ag facts)

Iowa has some of the best soil in the nation.
Iowa is called “The Bread Basket of the World” due to its highly productive soil.
It takes 500 years to produce just under an inch of topsoil, this is the most productive layer of soil where plants and many microorganisms inhabit.
There are more microorganisms in a handful of soil than there are people on earth.

Extension Activities

Explore the idea of how would the experiment by see whether the temperature of the water affects the soil’s retention level. Use the same steps, but have the water at a different temperature.
Mechanical Analysis of Soil (Jar Test) Find the makeup of local soil and water holding capacity.
- Materials:
  - Various soil samples 1 one-quart canning jar (with lid and ring) for each sample
  - Calgon water softener
  - Ruler (metric)
  - ½ cup measuring cup
  - Tablespoon Masking tape and pen (or similar materials for labeling jars)
- Procedure:
  - Place approximately ½ cup of loose (clod- and rock-free) soil in a quart jar. Add 1 heaping tablespoon of Calgon and 3 ½ cups of water. Cap and shake for five minutes (alternately inverting the jar will suffice). Allow the jar to sit, undisturbed, for at least 24 hours.
  - At the end of 24 hours, measure the depth of settled soil. This represents the total depth of soil. Shake thoroughly for five minutes (again, alternately inverting the jar). Let the jar sit, undisturbed, for 40 seconds. Now measure the depth of the settled soil with a ruler. This is the sand layer.
  - At the end of 30 minutes, measure the depth of the settled soil again. From this depth, subtract the thickness of the sand layer to obtain the depth of the silt layer above it.
  - The remaining unsettled particles in suspension represent the clay fraction and can be obtained by subtracting the depths of the sand and silt layers from the total depth determined in step 2.
  - Refer to the soil triangle and determine the textural class of your soil.

Suggested Companion Resources

Soybean Growth and Management Quick Guide, North Dakota State University
Step-By-Step Experiments With Soil; Gina Hagler

Sources/Credits

Author(s)