mineral, constituents
The mineral constituents of soils may be grouped on a basis of size of particles. Sand and coarser particles together with silt are primary minerals. The silt size is between 2 and 50 microns in diameter. Primary minerals are the minerals which change very little under the influence of weathering action. In soils, they include the complex silicates and simple compounds, mostly oxides (quartz or silicon dioxide). The primary minerals amount to 70% or more of the soil mass. Secondary or Clay Minerals. Below the silt size-range is the clay fraction, ranging in size from two microns and less. The clay portion consists of the complex aluminum and iron silicates. They are known as clay minerals or secondary minerals. These minerals have high colloidal properties, that is, their inner surface or sum of the individual individual particles is very great (several acres to one cubic foot of soil). Soluble, chemically active substances accumulate at the surfaces of these particles from the surrounding moisture, and the result is a great intensification of chemical processes. This is one important chemical property of soils.
