Soil the mineral components of soil; it consists of rock fragments, primarily quartz particles, ranging in size from in diameter. Silt ranges in size from . Clay cannot be resolved by optical microscopes as its particles are or less in diameter and a thickness of only 10 angstroms (10 m). In medium-textured soils, clay is often washed downward through the soil profile (a process called eluviation) and accumulates in the subsoil (a process called illuviation). There is no clear relationship between the size of soil mineral components and their mineralogical nature: sand and silt particles can be calcareous as well as
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