How to Read a Soil Texture Triangle Chart

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The US Department of Agriculture classifies soil texture according to its composition, and the three basic materials that comprise soil are clay, sand, and silt. The proportions of each of these components determines whether soil is classified as "sandy clay," "silty clay," "loam," or another term.

Traditionally, the USDA uses a soil triangle chart to represent the range of soil compositions. If you've never seen the chart before, it may be a little confusing to figure out. This article will show you step by step how to read the chart to determine soil texture.

Things You'll Need

• You will need to know the percents of clay, sand, and silt in a soil sample. If you just want to practice reading the chart, use arbitrary numbers.

Instructions

1. 

   Look at the orientation of the numbers on the side of the chart to understand in which directions you should read the percentages.

   On the left side, the numbers correspond to the percentage of clay and are read left to right. On the right side of the chart, the number correspond to the percentage of silt and are read from the top right to the bottom left. At the bottom of the triangle chart are the percentages of sand, and these are read from the bottom right to the top left.

   As an example to use in the following steps, let us assume that we are classifying the texture of a soil sample that is 30% clay, 20% silt, and 50% sand.

2. 

   Find the clay percentage on the left side of the triangle, and at that level, draw a straight line going left to right. See image.

3. 

   Next, find the silt percentage on the right side of the soil chart, and at that percentage level, draw a straight line from the top right to the bottom left. See image.

4. 

   Now, find the sand percentage at the bottom, and draw a straight line from the bottom right to the top left. See image.

5. Find where the three lines intersect, and the that is the soil texture. In this example, a soil sample that is 30% clay, 20% silt, and 50% sand would be classified as "sandy clay loam."

   You may have noticed that you don't need to use all three numbers to find the intersection point on the soil triangle. If you know any two of the numbers, you can always determine the soil texture.