Most productive soils are said to contain a mixture of clay, organic matter, sand and silt. If the soil contains at least 40% clay, it is called clay soil. It is a naturally occurring inorganic component of soil. They are alumino silicates containing potassium, sodium, calcium, magnesium or iron in small amounts while sand is silicon dioxide. They have many properties that make them very useful as a building material, in making earthen wares and many other applications in agricultural and industrial sector.
They are actually tiny particles less than 0.002mm in diameter. It is an important constituent of soil because it acts as a good binding agent between soil particles. It gives soil elasticity and cohesion. This makes the soil highly water retentive. Thus the soil holds more water and nutrients than other kinds of soils. Clay soils are fine textured as against sand that is coarse textured. As they are very fine, they align and bind with each other sliding against each other without much friction between them. This is the reason why this soil surface feels slippery.
Clay retains its shape when molded. This property is known as clay’s plasticity. When heated they become rock hard. It could be of three types based on the temperature at which the clay is baked or fired to reach its maximum hardness – earthenware, stoneware and kaolin.
The purest form of clay is kaolin or china clay. It is high fire and the main constituent of porcelain. The size of the particles is larger than the other two kinds of clay. At a very high temperature like 1800 °C, this kind can be converted into glass (vitrification). Stoneware is a mid fire clay hard and durable clay used for making dinner wares. Earthen ware or common clay is low fire that contains minerals like iron oxide which is used in the making of terra cotta pots and roofing tiles.