Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz (made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2) and feldspar (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8).
Silt particles range between 0.0039 to 0.0625 mm or 3.9 and 62.5 microns.
Silts are formed by physical weathering.
Coarse silts are chemically inactive.
Finer silts, which approach colloidal sizes, may exhibit some of the characteristic properties of clay.
The particles adhere to one another, and have a large surface area giving them a capacity to hold some water and nutrients.
Clays are formed from thin plate-shaped particles held together by electrostatic forces, so present a cohesion.The clay textures may be divided into light, medium or heavy depending on their clay content.
The clay fraction differs from the sands and silts in that it is composed predominantly of minerals formed as products of secondary weathering, i.e. the rock minerals undergo change before becoming clay .
Clay particles are usually colloidal (i.e.: may be as small as or smaller than 0.00005mm) and electrically charged. As a result of this they attract and hold water and plant nutrients.
Clay particles have a large surface area per unit volume and can hold more water and nutrients on these surface areas.
Changes in water content of some clays give rise to swelling and shrinkage of soil volume. This can be seen by the presence of cracks which develop as the clay soil dries and shrinks. Add enough moisture and the cracks close, i.e.: the clay swells.
Clay may exhibit the property of plasticity.
Clay particles are cohesive. For instance, it is difficult to cultivate moist clay soils and when dry, the clods are difficult to break up (for comparison, bricks are made of oven dried clay).