Soil, the unconsolidated rock fragment formed from weathering, may be classified or grouped on the basis of texture, or the size and shape of the soil particles. There are four major textural classification of soil:
1. Gravel: Rock fragments between 2 and 75 mm in size
2. Sand: Rock particles larger than 0.05 but less than 2 mm in size
3. Silt: Fine, Powderlike particles larger than 0.002 but less than 0.05 mm
4. Clay: Very small particles, less than 0.002 mm in size.
The term loam is also used for a combination of silt and sand that also contains organic material and that is suitable for the growth of plants or crops. Clay differs from gravel, sand, and silt not only in size, but in shapes and mineral composition. Gravel, sand, and silt consist of relatively coarse grained, bulky particles; the very finely divided clay particles are platelike in shape and have a strong affinity for water.