Soil can be defined in many ways. In civil engineering, soil is a naturally occurring, loose/un-cemented/weakly cemented/relatively unconsolidated mineral particles, organic or inorganic in character, lying over the bed rock which is formed by weathering of rocks. Soil is formed by different particels such as gravel, rock, sand, silt, clay, loam and humus.

Sand Silt and Clay.

Fig 1: Sand Silt and Clay.

In this article we discuss on properties, characteristics, size, shape and constituents of  least sized soil types namely sand, silt, clay and loam.

1. Sand

It is most extensively used construction material. It consists of particles of rock and hard minerals, such as silicon dioxide. They are the largest type of soil particles, where each particle is visible to naked eye. The large, relatively stable sand-particle size increases soil aeration, improves drainage in tight soils and creates plant-growth supporting qualities, or tilt.

Sand

Fig 2 : Sand

The particle size of course sand ranges from 2 – 4.75mm, Medium sand ranges from 0.425 – 2 mm and fine sand ranges from 0.075 – 0.425 mm. The bigger particle size of the sand gives wet or dry sandy soil a grainy texture when you rub it between your fingers, and it makes the soil light and crumbly even when you try to stick it together in your hand. The particle shape is angular, sub angular, rounded, flat or elongated. The texture is rough, smooth, or polished.

2. Silt

Silt is a sediment material with an intermediate size between sand and clay. Carried by water during flood it forms a fertile deposit on valleys floor. The particle size of silt ranges from  0.002 and 0.06 mm.

Silt in wet condition

Fig 3: Silt in wet condition

Silt is a non plastic or low plasticity material due to its fineness. Due to its fineness, when wet it becomes a smooth mud that you can form easily into balls or other shapes in your hand and when silt soil is very wet, it blends seamlessly with water to form fine, runny puddles of mud.

3. Clay 

Clay particles are the finest of all the soil particles, measuring fewer than 0.002 mm in size. It consists of microscopic and sub-microscopic particles derived from the chemical decomposition of rocks. Clay is a fine grained cohesive soil. They stick together readily and form a sticky or gluey texture when they are wet or dry.

 Clay when mixed with water

Fig 4: Clay when mixed with water

Clay is made of over 25 percent clay, and because of the spaces found between clay particles, clay soils hold a high amount of water. Clay expand when in contact with water and shrink when getting dry. Compared to sand particles, which are generally round, clay particles are thin, flat and covered with tiny plates. Organic clay is highly compressible and its strength is very high when dry, which is why it is used in construction as mud mortar.

4. Loam 

Loam is a mixture of clay, sand and silt and benefits from the qualities of these 3 different textures, favouring water retention, air circulation, drainage and fertility. These soils are fertile, easy to work with and provide good drainage. Depending on their predominant composition they can be either sandy or clay loam.

Loam

Fig 5: Loam

The way the other particles combine in the soil makes the loam. For instance, a soil that is 30 percent clay, 50 percent sand and 20 percent silt is a sandy clay loam, with the soil types before “loam” listed in the order their particles are most dominant in the loam. The labels “clay loam,” “silt loam” and “sand loam” are used to refer to soils that are composed predominantly of those ingredients.

Read More: Soil Structures and its Types