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The 1938 soil taxonomy divided soils into three orders dependent on dominant soil forming factors.

Intrazonal soils

Intrazonal soils have more or less well-defined soil profile characteristics that reflect the dominant influence of some resident factor of relief or parent material over the classic zonal effects of climate and vegetation. There are three major sub-types, two of which have two further sub-types each.

Calcimorphic or calcareous soils develop from a limestone. It has two sub-types:

Rendzina soils are thin soils with limited available water capacity.

Terra rossa soils are deep red soils associated with higher rainfall than rendzina.

Hydromorphic soils form in wetland conditions. There are two sub-types:

Gley soils - These occur when the pore spaces between the grains become saturated with water and contain no air. This lack of oxygen leads to anaerobic conditions which reduce the iron in the parent rock. This gives the soil a characteristic grey/blue colour with flecks of red.

Peat soils form under circumstances that prevent the breakdown of vegetation completely.

Halomorphic soils form due to soil salination.

Azonal soil

These soils are formed in mountainous regions out of fine grains produced by weathering. However, due to various reasons, this fine grained material constantly slides down the slope. As a result, the time necessary for the formation of soils does not become available. Therefore, these soils remain immature. An example is soil along the slopes of the Himalayan mountains. In river plains, particularly in flood-plain areas, new alluvium is deposited every year. The time for soil formation remains inadequate. Hence, flood plain soils also remain immature. In river plains, due to alluvium and availability of water, the farmlands are fertile but the soils remain immature.

See also

• Red Mediterranean soil

References

• Baldwin, M.; C.E. Kellogg; J. Thorp (1938). "Soil Classification". Soils and Men: Yearbook of Agriculture 1938. U.S. Government Printing Office, Washington, D.C. pp. 979–1001.
• Brevik, Eric C. (November 2002). "Soil Classification in Geology Textbooks" (PDF). Journal of Geoscience Education. 50 (5): 539–543. doi:10.5408/1089-9995-50.5.539. S2CID 116487861. Retrieved 2006-04-06.

v t e Soil classification
World Reference Base for Soil Resources (1998–)	Acrisols Alisols Andosols Anthrosols Arenosols Calcisols Cambisols Chernozem Cryosols Durisols Ferralsols Fluvisols Gleysols Gypsisols Histosol Kastanozems Leptosols Lixisols Luvisols Nitisols Phaeozems Planosols Plinthosols Podzols Regosols Retisols Solonchaks Solonetz Stagnosol Technosols Umbrisols Vertisols
USDA soil taxonomy	Alfisols Andisols Aridisols Entisols Gelisols Histosols Inceptisols Mollisols Oxisols Spodosols Ultisols Vertisols
Other systems	FAO soil classification (1974–1998) Unified Soil Classification System AASHTO Soil Classification System Référentiel pédologique (French classification system) Canadian system of soil classification Australian Soil Classification Polish Soil Classification 1938 USDA soil taxonomy List of U.S. state soils List of vineyard soil types
Non-systematic soil types	Sand Silt Clay Loam Topsoil Subsoil Soil crust Claypan Hardpan Gypcrust Caliche Parent material Pedosphere Laimosphere Rhizosphere Bulk soil Alkali soil Bay mud Blue goo Brickearth Brown earth Calcareous grassland Dark earth Dry quicksand Duplex soil Eluvium Expansive clay Fill dirt Fuller's earth Hydrophobic soil Loess Lunar soil Martian soil Mud Muskeg Paleosol Peat Prime farmland Quicksand Serpentine soil Spodic soil Stagnogley Subaqueous soil Takir Terra preta Terra rossa Tropical peat Yedoma
Types of soil

• Soil in the United States
• Pedology
• 1938 in the United States
• Soil science