Reading: Soil Texture and Composition

A field of dirt with no vegetation

Figure 1. A loam field.

The inorganic portion of soil is made of many different size particles, and these different size particles are present in different proportions. The combination of these two factors determines some of the properties of the soil.

  • A permeable soil allows water to flow through it easily because the spaces between the inorganic particles are large and well connected. Sandy or silty soils are considered ‘light’ soils because they are permeable, water-draining types of soils.
  • Soils that have lots of very small spaces are water-holding soils. For example, when clay is present in a soil, the soil is heavier, holds together more tightly, and holds water.
  • When a soil contains a mixture of grain sizes, the soil is called a loam (figure 1).

When soil scientists want to precisely determine soil type, they measure the percentage of sand, silt, and clay. They plot this information on a triangular diagram, with each size particle at one corner (figure 2). The soil type can then be determined from the location on the diagram. At the top, a soil would be clay; at the left corner, it would be sand, and at the right corner it would be silt. Soils in the lower middle with less than 50% clay are loams.

Illustration showing approximate percentages of clay, sand, and silt for various materials. Clay is 60 to 100 percent clay, 0 to 40 percent silt, and 40 to 60 percent sand. Sandy clay is 40 to 60 percent clay, 30 to 65 silt, and 60 to 100 percent sand. Silty clay is 40 to 60 percent silt, 50 to 70 percent clay, and 20 to 40 percent sand. Clay loam is 30 to 40 percent clay, 60 to 80 percent silt, 30 to 60 percent sand. Sandy clay loam is 20 to 40 percent clay, Loam is around 30 percent clay, 80 to 90 percent silt, and 50 to 60 percent sand. Silty clay loam is 60 to 75 percent silt, 35 to 40 percent silty clay loam, and 0 to 30 percent sand. Sandy loam is around 20 percent clay, 50 to 70 percent sand, and 80 to 100 percent silt. Loamy sand is around 15 percent clay and 70 to 80 percent sand. Sand is 90 to 100 percent sand and 0 to 10 percent clay. Silt loam is 75 to 85 percent silt, 20 to 50 percent sand, and 0 to 30 percent clay. Silt is 90 to 100 percent silt, 0 to 20 percent sand, and 0 to 20 percent clay.

Figure 2. Soil types by particle size.

Using the chart as a guide, what is the composition of a sandy clay loam? If you would like to determine soil type by feel, here’s a chart from the USDA to help you.


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