The size of individual particles, or grains, of soil is the most important aspect when it comes to soil classification
How do you describe and classify soil in the lab?
The most important aspect of soil classification is the size of the individual particles, or grains, of soil. A soil can be made up of gravel, sand, silt, clay, or any mixture of these, with each being progressively smaller. In order to test a soil sample, a number of sieves with increasingly narrow openings are stacked on top of each other and the particles are allowed to pass through until they reach a size that they cannot fit through. The mass of particles in each sieve is then measured and a grading curve is produced, indicating the percentage of material passing through each sieve. This curve can then be used to classify the material as well-graded, with a wide range of particle sizes, uniform, with a narrow range of sizes, or poorly graded, with an uneven spread of sizes.
Soils can also be tested in a lab for other properties such as their consistency and compactness, and are often also described, based on observation, with regards to their colour and particle shape. If the area that the sample is taken from has a known history, then the soil can also be described in terms of its origin.