The soil is the medium in which plants grow. It consists of mineral matter derived from the underlying rocks and organic matter mainly derived from the vegetative cover. The soil is not everywhere the same, its characteristics vary in both vertical and horizontal directions. Almost everyone is aware that soils have different colours and that there is a great difference between sand and clay, but few people are fully aware of the geographical distribution and significance of these differences. This is not surprising since it is only within recent years that soil scientists themselves have begun to study these matters intensively. As the results of their work unfold they are being adopted by geographers and used to amplify geographical description and provide further explanation of geographical problems.
The Soil Profile
All soil description is based upon observation of the soil profile which is the face of a vertical exposure from the surface down to the unaltered parent material. Such a face is seen to be composed of zones or horizons having different colours as well as other physical characteristics.
1. An accumulation of organic debris upon the surface, in various stages of decay. This is particularly noticeable in forest areas and usually absent in grassland. Soil scientists designate it as the A0 horizon.
2. A dark coloured horizon usually containing a relatively high amount of organic matter. This is known as the A 1 and is well developed in grassland soils.
3. A light coloured, leached horizon, well developed under forest vegetation but obscure or lacking in grassland. It is designated A 2.
4. The B horizon in which the material leached from the upper horizons is deposited. It is usually some shade of brown in colour and is somewhat more compact than the A horizon.
5. The C horizon or parent material. Because of minor differences soil descriptions often record other horizontal subdivisions such as A 3, B 1, B 2, etc.
Pedogeography or soil geography in its modern sense began with the observations of Dokuchaev, a Russian scientist who was sent out by the Imperial government in the 1870's to classify land for new settlement. He was impressed by the great regional differences in soil qualities throughout the empire. The great grasslands or steppes of southern Russia were underlain by chernozem, rich black soil which when cultivated yielded good crops of wheat. The forests of the north, on the other hand, grew upon grey soil, podzol, which had little natural fertility and required a great deal of careful attention in order to produce rye and potatoes while wheat could not be grown successfully. Dokuchaev found that black soils were formed on many types of bedrock or parent material and that the same rocks might be found under the podzols. It was clear to him that soils were to be correlated with vegetation and climate rather than with bed rock. It was true, of course, that the mineral matter or skeleton of the soil was derived from the bed rock but, provided that enough time has elapsed, all soils formed under the same climatic influences eventually came to be much alike in colour, organic matter content, chemical reaction and natural fertility. This geographic theory of soil distributions was accepted and elaborated by a number of pupils and co-workers of Dokuchaev and has eventually become familiar to soils men and geographers the world over.