THE SOIL TRIANGLE

The USDA classifies soil types according to a soil texture triangle chart which gives names to various combinations of clay, sand, and silt. The Soil Triangle is a commonly used visual representation of the possible soil type combinations based on soil particle size.  Larger particles will have less surface area per unit of mass or weight (and usually per unit of volume.  This means that soils composed of larger particles will have more open space (porosity) and will interact less (permeability) with water, air, and nutrients in the soil as compared to soils composed of smaller particles such as clay.

How to Use the Soil Triangle

  1. 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.
  2. 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%.

For More Information

USDA Soil Texture Calculator 

Plant and Soil Sciences eLibrary – Soils and Soil Water

SC-NRCS & ESRI-SC Partnership – Soil Physical Properties

USDA Soil Triangle