The soil is comprised of inorganic (90-95 per cent) and organic matter (5-10 per cent). Besides these soil contains water and air. The composition of the soil varies considerable depending on the type of soil.
Inorganic Components of Soil: Most of the inorganic components of soil exist as silicate minerals. The common elements in the soil are oxygen (46.6 per cent), silicon (27.7 per cent), aluminium (8.1 per cent), iron (5.0 per cent), calcium (3.6 per cent), sodium (2.8 per cent), potassium (2.6 per cent) and magnesium (2.1 per cent). Commonly found silicates in the soil are quartz (SiO2), Orthoclase (KAlSi3O8); albite (NaAlSi3O8) and epidote [4Ca0.3(AlFe)2 O3 6SiO2 H2O]. Many soils contain iron oxides, FeO (OH), magnetite (Fe3,O4). Certain soils contain titanium, and manganese oxides. Calcium carbonate is another common component of soil.
Soil also contains a mixture of minerals as carbonates, sulphates, chlorides and nitrates. Some soils also contain clay mineral, which are hydrated iron and aluminum silicates. These clay minerals bind cations such as Ca2+, Mg2+, Na+, K+, NH4+ so that these cations are not leached by water and are available to plants as plant nutrients.