1938 USDA soil taxonomy

The 1938 USDA soil taxonomy was a soil classification system adopted by the United States Department of Agriculture, now obsolete. The classification system used previously was developed and published in 1936 by C.F. Marbut, who was chief of the U.S. Soil Survey at that time. A drastic modification, the 1938 system was extensively revised in 1949 and remained in use until 1965. See USDA soil taxonomy for the current system.

Overview

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

Zonal soil

Marbut's Pedocal/Pedalfer boundary lies near the 98 meridian and 30 inches (760 mm) annual precipitation. (after Marbut, 1935)

Zonal soils have well-defined soil profile characteristics due to the influence of climate, organisms, and particularly, vegetation. Pedalfers and pedocals are subdivisions within the zonal soil order. Examples of zonal soils are the latosol or the podsol (sometimes spelled podzol).

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 3 major sub-types, 2 of which have 2 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 soilss 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. For eg,soils along the slopes of Himalaya mountains. In river plains, particularly in flood-plain areas, new alluvium gets 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, and 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". Journal of Geoscience Education 50 (5): 539–543. http://www.nagt.org/files/nagt/jge/abstracts/Brevik_v50n5p539.pdf. Retrieved 2006-04-06.