Soil texture is a physical property of soil that significantly influences how water and air move through the ground, affecting everything from drainage to nutrient availability. This characteristic is determined by the relative proportions of the three primary soil separates: sand, silt, and clay. The soil texture triangle is the standardized tool used by soil scientists and agricultural experts to classify a soil sample into one of twelve textural classes based on these measured percentages. Knowing the specific textural class is a practical first step for any decision related to land management, farming, or gardening.
The Three Components of Soil Texture
Soil texture begins with the size of the mineral particles that make up the material. Sand particles range from 2.0 millimeters down to 0.05 millimeters in diameter according to the United States Department of Agriculture (USDA) classification system. Sand particles feel gritty to the touch and create large pore spaces in the soil, which leads to rapid drainage and low water retention.
Silt particles are medium-sized, falling between 0.05 and 0.002 millimeters in diameter. When rubbed between the fingers, silt feels smooth and floury, lacking both the grittiness of sand and the stickiness of clay. Silt soils generally have moderate water-holding capacity and drainage characteristics, placing them between the two extremes.
The smallest particles are clay, measuring less than 0.002 millimeters. Clay particles are flat, plate-like structures, which gives them an extremely high total surface area. Clay soil feels sticky when wet and holds a significant amount of water and nutrients, though it also drains very slowly.
Understanding the Triangle’s Geometry
The soil texture triangle is an equilateral triangle where each of the three sides represents the percentage of one of the soil separates, ranging from 0% to 100%. The sum of the percentages for sand, silt, and clay in any soil sample must always equal 100%. A single point within the triangle represents a unique combination of the three components.
Reading the triangle requires understanding the directional lines for each component. The percentage of clay is read along the vertical axis, which is typically the left side, with its corresponding guide lines running horizontally across the triangle. Sand is read along the bottom axis, with its guidelines running diagonally upward and to the left.
The percentage of silt is read along the right side of the triangle, with its guide lines running diagonally downward and to the left. The triangle is divided into twelve distinct regions, each labeled with a specific textural class, such as “loam” or “sandy clay.”
Step-by-Step Guide to Plotting Texture
It is often easiest to start with the clay percentage, found on the left side of the triangle. Locate the given clay percentage on this axis and then trace a line horizontally across the triangle.
Next, focus on the silt percentage, which is found along the top right side of the triangle. Locate the measured silt value on this scale and follow the corresponding line diagonally downward and to the left. This line will intersect the horizontal line traced for the clay percentage at a specific point.
To confirm the accuracy of the intersection, use the third component, the sand percentage, found along the bottom base. Locate the sand value on this bottom axis and trace a line diagonally upward and to the left. A correct plot will have all three lines converge precisely at the same single point. The final step is to identify the name of the textural class region in which the intersection point falls.
Applying the Guide with a Practical Example
Imagine a soil sample has been analyzed, revealing a composition of 40% sand, 40% silt, and 20% clay. The plotting process begins by finding the 20% mark on the clay axis. From this 20% mark, follow the guide line straight across the triangle horizontally.
The next step uses the 40% silt value, which is located on the right-hand axis. Follow the line that corresponds to 40% silt, tracing it diagonally downward and to the left until it crosses the 20% clay line. These two lines will intersect at a point near the center of the triangle.
Finally, the 40% sand value is used as a check, found along the bottom axis. The line corresponding to 40% sand runs diagonally upward and to the left, and it should meet the other two lines at the same point. This intersection point falls squarely within the region labeled “Loam,” which is the resulting textural class for the sample. Loam indicates a relatively even balance of the three particle sizes, providing good water retention and drainage.