COLLECTION of values or a mathematical function describing the relative quantity, USUALLY by mass, of particles present by size.Usually, considerable energy is needed to disintegrate dirt, etc. particles into the PSD, which is then called a distribution of grain size.The sizes of soil-forming particles differ over a large range.Soils, depending on the prevailing size of particles within the soil, are commonly called GRAVEL, sand, silt, or clay.Gravels are rock FRAGMENTS of occasional quartz, feldspar, and other mineral particles. Particles of sand are COMPOSED mainly of quartz and feldspar.