AGRISCIENCE EXERCISE

University of Arkansas - AgriScience Project

AGRISCIENCE EXERCISE

ENVIRONMENTAL RESOURCE SYSTEMS

Key Concept: Soil & Land Resources

Sub-Concept: Basic Soil Properties

Agricultural Context: Different uses for different soils

Exercise: Determine Soil Texture by Separation and Ribbon Methods

Applied Principle: Differences in soil texture are found in different horizons (layers) in the soil. Soil texture affects its productivity and possible uses. The soil horizon can be identified by determining differences in texture.

Goals:

Discuss significance of soil texture.
Estimate soil texture using ribbon method.
Calculate soil texture using separation method.

Materials:

Soil samples (1 per group)
Water
2 - large graduated cylinders
Texture Triangle
Stopwatches (or watches with second hands)
Caps for cylinders
Data Sheets (1 per group)
Instruction Sheets (1 per group)

References: South Dakota AgriScience Lab Manual. (1994).

Procedures:

1. Divide the class into small groups of about 3 members each.

2. Provide each group with the appropriate materials and soil samples. [NOTE: Groups may use samples they previously gathered, and/or samples collected by the instructor; the instructor should be sure that each group has a different soil texture.]

3. Allow students to complete the exercise, offering assistance and guidance as needed.

4. When all groups have finished, conduct a class discussion regarding the exercise questions from their data sheets. Be sure to emphasize the agricultural applications of the concepts covered.

AGRISCIENCE EXERCISE

- Determine Soil Texture by Separation and Ribbon Methods -

Student Data/Instruction Sheet

INSTRUCTIONS: Your group is assigned to complete the following steps in determining soil texture using two different methods. After you have completed both parts of the exercise, answer the discussion questions. Be prepared to participate fully in a class discussion of these questions.

1. Gather all needed materials from instructor.

2. Pour the soil sample into one of the graduated cylinders to the 20 ml. line.

3. Shake the cylinder for 3 minutes. [Be sure to cap it first!]

4. Let the mixture stand for exactly 30 seconds, allowing the sand to settle.

5. Carefully pour off the liquid, making sure not to disturb the settled sand, into the second graduated cylinder.

6. Read the amount of sand and record the information on your group's Data Sheet.

7. Clean up the first graduated cylinder.

8. Allow the sample to set for 30 minutes. In the mean time, you will complete another test of soil texture -- the "Ribbon Method".

9. Soil scientists often find it necessary to estimate soil texture in the field. In this case, they use the Ribbon, or Texture-by-Feel method. To practice this, follow the steps below. [Take note that the feel of a soil may be influenced by the amount of moisture and organic matter present, so be cautious in your final decisions.]

Record the results of each step in the process in the "Texture by Ribbon Method" table provided.

a. Moisten a sample of soil the size of a golf ball, but don't get it very wet.

b. Work the sample until it is uniformly moist, then squeeze it out between the thumb and forefinger to try to form a ribbon (see Figure 1).

c. Your first decision is to determine if the moist soil is (see Figure 2):

(1) Extremely sticky and stiff; = One of the CLAYS

ribbons are over 2" long

(2) Sticky and stiff; ribbons 1 - 2" long = One of the CLAY LOAMS

(3) Soft, easy to squeeze, only slightly = One of the LOAMS

sticky; ribbons less than 1" long

d. Your second decision is to determine whether you need an adjective to refine your first description (see Figure 3):

(1) The soil feels very smooth = Use adjective SILT/SILTY

(2) The soil feels somewhat gritty = Use no adjective

(3) The soil feels very gritty = Use adjective SANDY

e. Your final refinement in this method is to consult the actual Soil Texture Triangle (see Figure 4):

(1) The lines on the actual texture triangle jog a little

(2) There are three additional, less common classes: sand, loamy sand, and silt

f. As a group, make a decision as to what the soil's texture actually is, and record this on your Data Sheet. (You will compare this to the results of the Separation Method later.)

Texture by Ribbon Method

Decision 1

Decision 2

Decision 3

Final Texture Decision

10. After 30 minutes, pour off the liquid from the second cylinder into the first.

11. Read the amount of silt and record the information.

12. Let the first cylinder set for 24 hours. Determine the amount of clay, and record the information.

13. Calculate the percent of each soil particle as follows:

10 ml. sand / 20 ml. sample = .50 / 100 = 50% sand

14. Using the Soil Texture Triangle, determine the texture of the sample you tested, and record on the Data Sheet.

Texture by Separation Method

Amount of Sand: __________ Percent Sand: __________

Amount of Silt: ____________ Percent Silt: ___________

Amount of Clay: ___________ Percent Clay: __________

Texture of Sample: _________________________________

15. Complete the following discussion questions. Be prepared to share your answers with the rest of the class.

a. Did your group arrive at the same answers for each method used on the samples? If not, why do you think the answers were different?

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b. Why is the texture (composition) of a soil important?

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c. What effect do you think soil texture has on how much water a soil can hold?

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d. Do you think that a soil's color has an effect on its texture? Why or why not?

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e. Why would a producer worry about the texture of soil in a field? Would it be easier to plow a clayey field or a sandy field?

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f. What do you think is the "ideal" soil texture for producing crops? What about for raising animals? Why?

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