University of Arkansas - AgriScience Project

AGRISCIENCE EXERCISE

PHYSICAL SCIENCE/ENVIRONMENTAL RESOURCE SYSTEMS

Key Concept: The Physical Environment / Soil & Land Resources

Sub-Concept: Understanding Soil & Water / Soil Water

Agricultural Context: Soil Water

Exercise: Observe Capillary Water Movement in Soils

Applied Principle: Moisture moves through soil in all directions, even against gravity, by capillary movement. The type of soil and its condition greatly affects how capillary water moves through it.

Goals:

  1. Define capillary water.
  2. Describe how capillary water moves through soil.
  3. Compare capillary movement of water in coarse, medium, and fine soil particles.


Materials: (per group)



References: Thompson, D.E. (1986). Science Laboratory Exercises for Vocational Agriculture Students. Office of the State Director for Vocational Education, Honolulu, HA.

Soil Conservation Service. (1986). Soil and Water Conservation Activities for Youth. Program Aid Number 1391. Washington, D.C.: USDA.

Teacher Preparation Notes:

Procedures for Conducting the Activity:

1. Divide the class into small groups of no more than three students each, and provide each with a data sheet and the necessary materials for this exercise.

2. Instruct the students to complete the activity as directed on their data sheets. You may wish to monitor their progress as they work; however, it is suggested that the students be left to follow the instructions and complete the activity on their own.

3. Once all groups have completed the exercise, discuss the answers to the discussion questions as a class. Be sure to make note of the practical agricultural applications of the principles demonstrated.

AGRISCIENCE EXERCISE

- Observe Capillary Water Movement in Soils -

Student Data/Information Sheet

1. As a group, read through the instructions before continuing.

2. Gather the following materials:





3. Mark 1-inch increments along the length of each cylinder.

4. Fasten cloth over one end of each cylinder, securing with rubber bands.

5. Fill each cylinder about 3/4 full of one of the following dry soils: sand, clay, topsoil.

6. Tap the cylinders slightly to settle soil.

7. Set one cylinder into the jar, cloth end down.

8. Pour water into the bottom of the jar (do NOT pour into cylinder). Using the watch, record the time it takes for the water to reach each increment mark on the data table below. Also note whether the water reaches the top of the cylinder.

9. Remove cylinder from jar. Pour out any remaining water in the jar.

10. Repeat steps 6 and 7 for the other two cylinders.































DATA TABLE

MARK

TIME FOR WATER TO REACH MARK (in seconds)

SAND CLAY TOPSOIL

1"

2"

3"

4"

5"

Top


11. Clean up your mess, and put away your materials (as directed by the instructor).

12. Answer the following questions regarding the exercise you just completed. Be prepared to share your results with the class.

a. Which soil exhibited the quickest water movement? Why?















b. Which soils, if any, carried the water all the way to the top of the cylinders? Why?















c. What is capillary water? Why is it an important aspect of soils?















d. What does this exercise tell us about how the water and rock cycles operate together?











TEACHER BACKGROUND SHEET

- Capillary Water Movement -

Moisture moves through soil in all directions, even against gravity, by capillary movement. This movement is caused by the attraction water molecules have for each other as well as the attraction between water molecules and soil particles. Water molecules cling together and form droplets in the air or on a greasy surface where there is nothing to interfere. But when a drop of water falls on soil particles, it spreads out as a thin film over the soil particles - because the attraction between the soil particles and the water molecules is greater than the attraction between the water molecules themselves. Water that moves through soil this way is known as capillary water.

How far and how fast capillary water will move in a soil depends on the size of the soil particles and the condition of the soil. If the spaces around the soil particles are large, the attraction between the water molecules and the soil particles will not be enough to overcome the weight of the water and it will not rise appreciably; however, what movement it makes will be rapid because there will be little friction. This is true in sandy soils.

On the other hand, in fine-textured soils the particles are closer together and the attraction between soil and water is greater. Water may then be expected to rise more slowly but higher in soils of fine texture.

Under field conditions moisture moves from wetter soil to drier soil. The difference is not always great, therefore capillary water moves slowly and not far. Even so, enough moisture moves a short distance to the roots of growing plants to make it an important plant-soil relationship.

Much soil moisture can be lost when capillary water moves to the surface and evaporates. Using mulches can reduce this.



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