<P> Sand and silt are the products of physical and chemical weathering of the parent rock; clay, on the other hand, is most often the product of the precipitation of the dissolved parent rock as a secondary mineral, except when derived from the weathering of mica . It is the surface area to volume ratio (specific surface area) of soil particles and the unbalanced ionic electric charges within those that determine their role in the fertility of soil, as measured by its cation exchange capacity . Sand is least active, having the least specific surface area, followed by silt; clay is the most active . Sand's greatest benefit to soil is that it resists compaction and increases soil porosity, although this property stands only for pure sand, not for sand mixed with smaller minerals which fill the voids among sand grains . Silt is mineralogically like sand but with its higher specific surface area it is more chemically and physically active than sand . But it is the clay content of soil, with its very high specific surface area and generally large number of negative charges, that gives a soil its high retention capacity for water and nutrients . Clay soils also resist wind and water erosion better than silty and sandy soils, as the particles bond tightly to each other, and that with a strong mitigation effect of organic matter . </P> <P> Sand is the most stable of the mineral components of soil; it consists of rock fragments, primarily quartz particles, ranging in size from 2.0 to 0.05 mm (0.0787 to 0.0020 in) in diameter . Silt ranges in size from 0.05 to 0.002 mm (0.002 to 0.00008 in). Clay cannot be resolved by optical microscopes as its particles are 0.002 mm (7.9 × 10 in) 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 siliceous, while textural clay (0.002 mm (7.9 × 10 in)) can be made of very fine quartz particles as well as of multi-layered secondary minerals . Soil mineral components belonging to a given textural class may thus share properties linked to their specific surface area (e.g. moisture retention) but not those linked to their chemical composition (e.g. cation exchange capacity). </P> <P> Soil components larger than 2.0 mm (0.079 in) are classed as rock and gravel and are removed before determining the percentages of the remaining components and the textural class of the soil, but are included in the name . For example, a sandy loam soil with 20% gravel would be called gravelly sandy loam . </P> <P> When the organic component of a soil is substantial, the soil is called organic soil rather than mineral soil . A soil is called organic if: </P>

Write a note on different kinds of soil