Soil Texture Classification

Illustration of relative soil particle sizes.

Soil texture is classified according to particle size. Clay has the smallest particle and pore space size, followed by silt, and then by sand with the largest size particles. Soil texture is very important for anticipating the potential infiltration, movement, and storage of soil water.

Soil triangle showing relationship between various soil textures.

The USDA soil triangle, based on the FAO soil triangle, is a reference tool that allows you to classify the soil based on the composition of its textures. For example, if you take a sample of soil and determine that it is roughly 40% silt, 40% sand, and 20% clay, then we can see that the soil classification is "Loam".

In-Depth:Soil Formation Process

Soil cross-section schematic showing soil horizons.

Soil formation is the result of many natural processes. Soils typically develop such that the top layers show the greatest impact of vegetation and weathering (the O and A horizons) while the bottom layers (the C horizon) have the least influence from plants and weathering.

Climate is the most influential factor in soil formation as it directly affects the weathering of source material and is critical for biological activity.

Other natural influences of soil formation include vegetation type and patterns, the local geology, biological activity, mineral reactions, and watershed topography. These will affect how much rainfall or snowmelt will enter the soil and how it will move or get stored within the soil.