Another factor that affects soil texture is if the particles are angular or rounded. How much they pack together, or have large pore spaces. What minerals are found in the soil sample? Usually, the texture may be fine, or coarse or something in between.
The texture of soil is generally caused by the size of the particles that make up the soil. There are three types of particles that make up soil -- sand, silt, and clay. Each of these is in a different size range. Sand, for example, is defined as particles from 2 mm to .02 mm while clay particles are less than .002 mm.
The soil is made up of many particles that have different sizes. We can notice this aspect when are rubbing a bit of soil between our fingers. The texture of soil is given by these particles.
For instance, for some soils, the number of large size particles is bigger than the number of small size particles. For other soils, the number of small size particles is bigger than the number of large size particles and in other soils the number of large size and small size particles is the same.The balanced soils are proper the most suitable for plants.
Soil "texture" is directly related to how the constituent soil materials behave and interact with water.
A smooth slippery soil likely will have a high clay content due to the fact that clay particles are generally flat like a penny. When water is present between clay particles, it causes them to stick together or be "cohesive" due to hydrostatic attraction.
A soil material that is crumbly even when moist will likely have a high silt content due to the fact that silt particles are generally round like a marble. When water is present between silt particles, they tend not to stick together due to the minimal surface area available when they "touch" each other.
Sand particles are generally rounded, sub-angular to angular in shape and therefore would behave like giant silt particles having little or no cohesive properties.
In the geotechnical engineering field, soil "texture" is a critical factor to consider when designing the foundations of buildings. Engineers must evaluate the soil type and select the foundation system that is best suited for either clay, silt, sand or combination soils.