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 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.

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 break down of vegetation completely.

Halomorphic soils form due to soil salination.

Azonal soil

Azonal soils are without well-developed characteristics due either to their youth or to some condition of relief or parent material which prevent soil development. Soils forming in recent eolian, alluvial and colluvial deposits are azonal.

References

Baldwin, M. (1938). "Soil Classification". Soils and Men: Yearbook of Agriculture 1938. U.S. Government Printing Office, Washington, D.C. pp. p. 979-1001. {{cite book}}: |pages= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

Brevik, Eric C. (2002). "Soil Classification in Geology Textbooks" (PDF). Journal of Geoscience Education. 50 (5): 539–543. Retrieved 2006-04-06. {{cite journal}}: Unknown parameter |month= ignored (help)

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