Soil Textural Triangle
Name: _________________________________________________ Period : _______ Date: _________
Essential Question: How do I classify the types of soil using the soil triangle?
A soil texture triangle is used to classify the texture class of a soil. The sides of the soil
texture triangle are scaled for the percentages of sand, silt, and clay.
Soil Texture is one of the first things
determined when a soil is examined. It is
related to weathering and rocks as parent
materials. The differences in horizons may be due to the
differences in texture of their respective parent materials
or rocks where soil formed.
First, look at the orientation of the percentages on the sides of the triangle. The numbers are arranged
symmetrically around the perimeter. On the left the numbers correspond to the percentage of clay, and
on the right the numbers correspond to the percentage of silt. At the bottom of the triangle chart are the
percentages of sand.
To classify a soil sample, you find the intersection of the
three lines that correspond the three proportions. On the
chart, all of the percents will add up to 100%.
Example: Classify a soil sample that is 30% clay, 15%
silt, and 55% sand. First locate 30% on the clay axis, and
draw a line horizontally from left to right. Next, locate 15%
on the silt axis, and draw a line going down diagonally to the
left. Finally, locate 55% on the sand axis, and draw a line
going up diagonally to the left. The intersection is in a region
called Sandy Clay Loam. See figure on the right.
In some environments clay aggregates form that are so strongly cemented together that they feel like fine
sand or silt. In humid climates iron oxide is the cement. Humid air can hasten the process of rusting, which an
example of chemical weathering. Chemical weathering occurs when the minerals within the rocks are
chemically altered. Oxidation represents a form of chemical weathering where oxygen combines with
elements in the rock to form oxides. Iron-rich rocks provide a simple example of this: Oxidation results in
a rusting effect similar to the rust found on exposed iron. In desert climates silica is the cement and in arid
regions lime can be the cement. It takes prolonged rubbing to show that they are clays and not silt
loams.Some soils derived from granite contain grains that resemble mica but are softer. Rubbing breaks down
these grains and reveals that they are clay. These grains resist dispersion and field and laboratory
determinations may disagree.
Clarifying Questions:
1. What is a soil texture triangle?
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2. What is one of the first things determined in doing soil analysis or examination?
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3. Describe the comparative sizes of gravel, sand, silt and clay.
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4. What is the soil parent material?
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5. How is soil classified?
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6. Why do farmers determine the texture and pH level ( acid 1-5, base 7-14) of their farm soil?
Think!_______________________________________________________________________
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7. How is iron oxide affected by humid climates?
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8. What is the cement in rocks in the desert?
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9. What is the cement in rocks in the arid or dry regions?
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Use the Soil Textural Triangle below to answer questions 10-13.
10. What type of soil is made of 10 % sand, 80 % clay, and 10% silt?
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11. What type of soil is made of 60% sand, 10% silt, and 30% clay?
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12. What type of soil is made of 50% sand, 40% clay,
and 10% silt?
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13. What kind of soil is made of 40% sand, 40% silt,
and 20% clay. It is the best soil for most plants.
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14. The three sides of the soil textural triangle are
represented by the percent (%) of sand, ___________,
and _____________
http://soils.usda.gov/education/resources/lessons/texture/ Video: http://www.youtube.com/watch?v=-KNcUNGaou0
http://www.had2know.com/garden/classify-soil-texture-triangle-chart.html
Answer Key:
10. clay
11. sandy clay loam
12. sandy clay
13. loam