One of the most important characteristics of soils is soil texture. Soil, by definition, consists of particles less than 2 mm in diameter. Soil texture is the proportion of sand, silt, and clay particles that are of soil size (2 mm or less). The US Department of Agriculture classifies soil particle sizes as the following: sand particles are the largest soil particles and range from 0.05 mm to 2.00 mm in diameter, clay particles are the smallest and are less than 0.002 mm in diameter, and silt-sized particles are in the middle. The Disovery Channel website actually has a good graphic to demonstrate the relative size of each particle size class:
Since gravel is larger than 2 mm in diameter, it is not considered a soil particle. Another good visual description is to consider a beach ball as a sand particle, a silt particle as a baseball, and a clay particle as a BB. That would be roughly the relative sizes of each particle if they were enlarged.
Soil scientists classify soil textures into 12 soil texture classes as shown in the soil texture triangle:
As an example, say we have a soil that is 45% clay. So we move up the left side of the Triangle until we get to the 45% line (numbers align with their respective lines). This soil is also 30% sand so we move from the right side to the left along the bottom axis until we get to the 30% sand line. The point where the 45% clay and the 30% sand lines intersect lies within the boundaries of the clay loam soil texture class. We can follow the silt percentage line to find that we have 25% silt. Since the sum of each should add to 100% we can also find the silt proportion by difference (100% – 45% – 30% = 25%).
Soil scientists measure soil texture by a variety of methods. The most common for everyday use is the texture-by-feel method which can be done using the following chart (click to enlarge):
It is just like any other dichotomous key with a series of yes/no questions. It doesn’t really give an exact percentage of the sand, silt, and clay, but gets the scientist “in the ballpark” of what the soil texture class should be. UC Davis put out a great video on how to do texture by feel:
Texture by feel is useful in the field. It does take lots of practice to “calibrate” your hands with soils of known textures. Some tips that I’ve learned along the way are the following:
- Mica in the sand-sized fraction can act like clay making long ribbons
- Silt feels like baking flour
- Expansive clays may lead longer ribbons and overestimation of clay content
Another method that is commonly used is the hydrometer method, which is a more reliable, repeatable, and unbiased method than texture by feel, thought it takes much longer. The Natural Resources Conservation Service (NRCS) made a great video on how to determine soil texture by the hydrometer method:
The chemical they use is a dispersant which chemically breaks up any soil peds or aggregates.
Soil texture is more than just how the soil feels to you or me. Soil texture is important because it is one of the principle properties of soils. Texture effects on how fast water can move through soil, erodability, retention of nutrients for plants and microbes, the microbiol community makeup, bulk density, and with all of those properties, what ecosystems or cropping systems can exist on that soil.
Since soil texture is so important, I’ll be referring back to this post regularly in future blog posts.
Colby Digs Soil 
why can’t we determine the silt of soil by feel method?
We do, sort of. The texture-by-feel method is more qualitative, and is used to place the soil in texture classes of the textural triangle. It’s difficult to get “exact” contents of sand, silt, or clay by the texture-by-feel method without lots of practice. However, if we place a soil into a textural class, we get sort of a range of contents of sand/silt/clay.
Further, the first few tests in the texture-by-feel flowchart determine if it’s a sand, loamy sand, or something else. Then we make a ribbon. If that ribbon is long enough we know there’s a high clay content. If the ribbon feels gritty we know there’s a lot of sand, thus we call it a sandy loam, sandy clay loam, or sandy clay. If it feels smooth, then we know there is a high silt content, and we call it a silt loam, silty clay loam, or silty clay. Does that answer your question?