The Earth Contents

5. Soils

There is a range of particle sizes in soil parent materials

Particle sizes are: 2 micrometre (µm) (clay), 2.50µm (silt), 50µm-2mm (sand), 2mm-2cm (gravel), > 2cm (rocks). The proportions of these different-sized particles determine the physical properties of the soil, which can vary from one soil type to another. This variation is ultimately dependent on the bedrock from which the different-sized particles arose. This is usually, but not always, the underlying rocks. Alluvial plains, for example, mostly consist of sediments transported from elsewhere by flood waters. It is these sediments which form the basis of their soils. In determining the chemical properties of the soil the clay components are key constituents, as many plant nutrients (e.g. potassium, phosphate) are present in the clay fraction.

Soil types as a function of percentage composition of silt, sand and clay particles

What are the range of particle sizes for clay, silt, gravel and rock?