Soil mechanics necessary to run water through the sieves to wash the coarse particles and clods through. A variety of sieve sizes are available. The boundary between sand and silt is arbitrary. According to the Unified Soil Classification System, a #4 sieve (4 openings per inch) having 4.75mm opening size separates sand from gravel and a #200 sieve with an 0.075 mm opening separates sand from silt and clay. According to the British standard, 0.063 mm is the boundary between sand and silt, and 2 mm is the boundary between sand and gravel. The classification of fine-grained soils, i.e., soils that are
Soil morphology Typical bulk density of clay soil is between 1.1 and 1.3 g/cm. This calculates to a porosity between 0.58 and 0.51. This seems counterintuitive because clay soils are termed "heavy", implying "lower" porosity. Heavy apparently refers to a gravitational moisture content effect in combination with terminology that harkens back to the relative force required to pull a tillage implement through the clayey soil at field moisture content as compared to sand. Porosity of subsurface soil is lower than in surface soil due to compaction by gravity. Porosity of 0.20 is considered normal for unsorted gravel size material at depths below