Difference Between Silt and Clay

Silt vs Clay

The word soil, when used in normal contents, just refers to that on which we all stand. However, engineers define (in construction) soil as any earth material that can be moved without blasting, while geologists define as rocks or sediments altered by weathering. Practicing engineers classify soil into different types based on the grain (particle) size distribution. According to this classification, the main soil types are boulders, gravel, sand, silt, and clay. Different ‘soil separate size limits’ have developed by different institutions and organizations like Massachusetts Institute of technology (MIT), US Department of Agriculture (USDA), American Association of State Highway and Transportation Officials (AASHO), Unified Soil Classification System, etc. However, currently the classification of Unified Soil Classification System is widely used all over the world. According to unified soil classification system, if particle sizes of soil is less than 0.075mm, they may either be silt or clay. Both clay and silt fall under the category of fine grained soil.


A particular soil is classified as clay when it contains clay minerals. Clays are plastic and cohesive. Clay particles cannot be seen through naked eye, but it can be seen through a powerful microscope. Kaolinite, montmorillonite, illite are mostly found clay minerals in soil. These are tiny plates or flake like structures. Clay minerals are very active electrochemically. When lots of clay minerals are found in a particular soil, then that soil is known as heavy or dense soil. In dry condition, clay is almost hard like concrete. The spaces between soil particles are very tiny. In soil mechanics, clay plays an important role as it has the ability of changing the chemistry or behavior of a given soil. Soils with clay minerals are commonly used to make or mould shapes and statues. Movement of plant roots, air and water through wet clay is very hard. The specific area of clay minerals is high (specific area = surface area: mass ratio)


Silt is fine grained soil with little or no plasticity. Clays can be further sub classified as organic silt and inorganic silt. Organic silt contains finely grained organic matters, while inorganic silts are not. Permeability of silt is low. That means drainage of water through silty soil is not easy. Silts mostly contain finer particles of quartz and silica. Silts are moisture sensitive; that is, small change in moisture will cause great change in dry density.

What is the difference between Silt and Clay?

Though silt and clay are classified as fine soils, they have some difference between them.

- Clay particles are much lesser in size than silt particles, even though all soils with particles size less than 0.075mm are classified as either silt or clay.

- Clay contains clay minerals,while silts do not contain clay minerals.

- Plasticity of clay is much more than that of silt.

- Surface texture of silt is smooth and slippery to touch when wet, while clay is sticky and plastic-like when wet.

- In most of the cases, dry strength of clay is greater than that of silts.

- Clays are energy sensitive to dry density, while silts are moisture sensitive to dry density.

- Dilation of silt is greater than that of clay.

- Toughness of clay is higher than silts.