CHAPTEE II 



THE ORIGIN OF SOILS 



THE principal object in describing the origin and 

 formation of soils is to account for the presence of the 

 various constituents physical and chemical, organic and 

 inorganic and to show how the supplies of plant food are 

 maintained. 



The soil is obviously derived from the material under- 

 neath. This frequently consists of solid rock, and the 

 connection between it and the soil can generally be traced 

 by examination of the intermediate layers. The rock is 

 usually cracked and fissured towards its upper limit, and 

 above this it is crumbled and broken into fragments ; these 

 are of various sizes, but gradually become smaller and 

 finally merge into soil. The topmost layer (some 6 or 

 8 inches deep) is usually of a darker colour owing to 

 the accumulation of organic matter. In cultivated soils 

 the 1-ine of demarcation is often very sharply defined, but 

 in other respects the surface layer and the subsoil are 

 usually much alike. 



In some cases the soil is formed, by comparatively slight 

 modification, from alluvium or drift materials which have 

 been transported from a distance the former by the action 

 of running water and the latter by glacial ice. These 

 deposits are, of course, derived from rocks, and may be 

 regarded as intermediate between them and the soil which 

 is the final product, but they are sometimes very thick and 

 widely spread, and are themselves regarded by geologists 

 as rocks. 



