Soil is comprised of mineral and organic components, as well as voids. The mineral portion of soil includes three size classes of particles: sand, silt, and clay. Collectively, sand, silt, and clay comprise a soil’s texture. For the average person, we discuss these size classes based on the United States Department of Agriculture (USDA) classification system. In that system, mineral soil is defined as particles less than 2.0 millimeters in diameter. Sand sized particles fall between 2.0-0.05 millimeters, silt ranges from 0.05-0.002 millimeters, and clay sized particles are less than 0.002 millimeters. Anything greater than 2.0 millimeters is defined as a rock fragment.
Organic matter typically comprises a relatively small volume of a soil, and is concentrated in the near-surface zone. The dark black and brown colors we associate with topsoil are a result of organic matter present in the soil. Topsoil is actually a general term used to describe organically enriched surface soil horizons (A horizons), but is not a term used in soil science directly.
The rest of a volume of soil is comprised of void spaces (pores) which contain either gases (atmosphere) or liquids (water). Although we sometimes overlook this portion of the soil, it is an absolutely critical component that has a tremendous effect on the development and properties of a particular soil.
By measuring the percent of sand, silt, and clay present in a sample of soil, one can determine its soil textural class using the USDA textural triangle. There are 12 texture classes on the triangle: sand, loamy sand, sandy loam, sandy clay loam, sandy clay, loam, silt loam, silt, silty clay loam, silty clay, clay loam, and clay.
SOIL TEXTURE EXERCISE
Want to get a rough idea of the texture of your soil? All you’ll need are a few things from around the house.
- A glass jar with lid (you can use an old spaghetti sauce or pickle jar)
- A ruler
- A clock or timer
- Grab a sample of your soil, enough to fill a third of the jar.
- Once the soil is in the jar, take an initial height measurement.
- Add water to the jar until it is about 3/4 full.
- Put the lid on the jar securely and shake the jar until the soil clumps break down.
- Measure the height of the sediment after about 40 seconds and write this value down on a piece of paper. This is your sand measurement.
- Measure the height of sediment again after 2 hours. This is your silt measurement.
- It could take several days for the rest of the soil particles (clay) to fall out of suspension, but since you measured the original height of soil, and you have the other two measurements, you can calculate the clay content without waiting that long.
What’s going on is that the sand particles will fall out of suspension first (at 40 seconds), then the silt particles fall out of suspension (2 hours), and finally all the clay particles drop out. This all happens based on a principle called Stoke’s Law. Stoke’s law basically states that heavier particles (sand) fall out of suspension faster than lighter particles (clay). This creates the layering effect you see in the jar.
To get the percent of sand, silt, and clay in your soil sample, use the following equation:
Sand height = first measurement
Silt height = second measurement – first measurement
Clay height = original measurement (before water was added) – second measurement
Percent sand = (sand height / original measurement)*100
Percent silt = (silt height / original measurement)*100
Percent clay = (clay height / original measurement)*100
Once you have these percentages, you can go to the NRCS soil texture calculator to input your percentages and determine your texture class. This method isn’t perfect but will get you in the ballpark of your soil’s texture class.