The soil scientist recognizes that all soils can be subdivided into three orders, known as zonal, intrazonal, and azonal orders. Zonal soils, formed under conditions of good soil drainage through the prolonged action of climate and vegetation, are by far the most important and widespread of the three orders. Intrazonal soils are simply those formed under conditions of very poor drainage (such as in bogs, flood-plain meadows, or in the playa lake basins of the deserts) or upon limestones whose influence is dominant.
Azonal soils have no well-developed profile characteristics, either because they have had insufficient time to develop or because they are on slopes too steep to allow profile development. Azonal soils include thin, stony mountain soils of the earth's mountain regions (lithosols), freshly laid alluvial materials, or dune sands (regosols). These usually have poorly developed profiles and cannot be easily classified, whereas the zonal and intrazonal soils have distinctive profile characteristics as the result of long development.
The great soil groups - The U.S. Department of Agriculture in 1938 recognized about 30 great soil groups, but these can be simplified into-about 18 principal varieties, including both zonal and intrazonal orders, that have worldwide distributions under similar climatic and geomorphic conditions.
The founder of modern theories of soil origin and classification was V.V. Dockuchaiev, a Russian geologist; his studies between 1882 and 1900 led him to the concept that soil is an independent body whose character is determined primarily by climate and vegetation. A Russian follower of Dockuchaiev, K.D. Glinka, expanded the concepts of horizons in the soil profile.
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