*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

Copy of the textural triangle for each student
Ruler or straight edge

### 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*