Residual soils are formed at the same site where the weathering of the parent rock has taken place or soils formed in situ from the underlying rocks. These are also called sedentary soils. In these soils the surface layers are most weathered but the degree of weathering decreases with increasing depth, and the rock fragments become progressively larger and mostly chemically unaltered until they finally integrate with the underlying parent rock.The red soils are poor in calcium, magnesium, phosphorus and nitrogen and coloured red by the presence of iron peroxide. The black soils, also called as black-cotton soils, are rich in clay, well supplied with potash,'calciunl, magnesium and iron. These are well suited to the cultivation of cotton and are characterised by the development of wide and deep cracks during the summer season.