Learning About Instruments and Protocols

Activity: Using the Textural Triangle


Purpose:

The textural triangle is one of the tools soil scientists use to visualize and understand the meaning of soil texture names. This activity will help students to see how the distribution of sand, silt, and clay in their soil samples are classified into names that allow them to describe the texture of the soil. The activity illustrates benefits of standards, systematic ordering and classification.

Overview:

Students will be instructed in how to properly use the textural triangle. They will also be given a sample set of sand, silt, and clay distributions to practice determining the textural class name with the triangle.

Level:

Time Needed: One class period

Key Concepts and Skills:

Students will develop concepts related to understanding how different particle sizes in the soil are distributed to create a specific texture. They will also develop the specific skill of reading information from a triangle diagram, as well as simple mathematical skills to estimate percentages that sum to 100.

Materials

Preparation

A discussion of different size particles in soils and their distribution should occur before this activity. See the description of soil texture and particle size distribution in Part III and in the Appendix.

Background

Sand, silt, and clay are the 3 particle sizes of mineral material found in soils. The amount of each of these is called the "particle size distribution" and the way they feel is called the "soil texture". Soil Scientists have created classes which break these textures into 12 categories. The textural triangle is a diagram which shows how each of these 12 textures are classed (CLASSIFIED?), based on how much sand, silt, and cly is in each. This activity will describe how to read the textural triangle and will help students to visualize the amount of each size particle they feel in their soil samples.

What to do and How to do it:

Preliminary Exercise for Younger Students

Younger students may not be able to use the triangle at first, but should still practice feeling the soil and becoming familiar with soil texture (sandy, silty, loamy, clayey).

Main Activity

Follow these steps to determine the name of your soil texture:

    1. Place a plastic sheet or tracing paper over the soil triangle

    2. Place the edge of a ruler at the point along the base of the triangle that represents the percent of sand in your sample. Position the ruler on or parallel to the lines which slant toward the base of the triangle.

    3. Place the edge of a second ruler at the point along the right side of the triangle that represents the percent of silt in your sample. Position the ruler on or parallel to the lines which slant toward the base of the triangle.

    4. Place the point of a pencil or water soluble marker at the point where the two rulers meet. Place the top edge of one of the rulers on the mark, and hold the ruler parallel to the horizontal lines. The number on the left should be the percent of clay in the sample.

    5. The descriptive name of the soil sample is written in the shaded area where the mark is located. If the mark should fall directly on a line between two descriptions, record both names.

Practice Exercises:

Use the following numbers to determine the soil texture name using the textural triangle. When a number is missing, fill in the blanks (note: the sum of %sand, silt and clay should always add up to 100%):


  % SAND     %SILT       %CLAY      TEXTURE NAME

 
a)        75        10        15             sandy loam

b) 10 83 7

c) 42 37

d) 52 21

e) 35 50

f) 30 55

g) 37 21

h) 5 70

i) 55 40

j) 45 10

Feel the texture of a moist soil sample in your classroom. Sand will feel "gritty", while silt will feel like powder or flour. Clay will feel "sticky" and hard to squeeze, and will probably stick to your hand. Looking at the textural triangle, try to estimate how much sand, silt, or clay is in the sample. Find the name of the texture that this soil corresponds to.

Older Students can:

Practice determining the percent sand, silt, and clay in their samples using the hand "texturing" method along with the textural triangle. Their estimates can then be verified with the procedure outlined in the protocol for "Particle Size Analysis" (settling experiment) which will tell them more quantitatively exactly how much of each size particle is in their sample.

Once students feel more confident in correctly estimating the texture, design a game, or competition to see which students come the closest in their estimation to the actual values as determined by the settling method.

Further Investigations:

Develop a set of "standard" soil texture samples which can be used for students to practice determining the texture of their soil. These standards should include one example of each of the twelve textural classes, with a %sand, silt, and clay listed that was determined by the settling method.

Evaluation:

Verify that students are understanding the relationship between particle size distribution by testing how well they can determine the textural class of unknown samples by feel. Use practice exercises, such as the ones given above to determine how well they can use the textural triangle.

Acknowledgement: Adapted from L.J. Johnson. 1979. Introductory Soil Science: A Study Guide and Laboratory Manual. MacMillan Pub. Co., Inc., N.Y.


v. April, 1996