The 1938 USDA soil taxonomy was a soil classification

Soil classification

Soil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use.- Overview :...

 system adopted by the United States Department of Agriculture

United States Department of Agriculture

The United States Department of Agriculture is the United States federal executive department responsible for developing and executing U.S. federal government policy on farming, agriculture, and food...

, 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

USDA soil taxonomy

USDA Soil Taxonomy developed by United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series.- Example of...

 for the current system.

Overview

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

Pedogenesis

Pedogenesis is the science and study of the processes that lead to the formation of soil ' and first explored by the Russian geologist Vasily Dokuchaev , the so called grandfather of soil science, who determined that soil formed over time as a consequence of...

.

Zonal soil

Zonal soils have well-defined soil profile characteristics due to the influence of climate

Climate

Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

, organisms, and particularly, vegetation

Vegetation

Vegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...

. Pedalfer

Pedalfer

Pedalfer is composed of aluminum and iron oxides. It is a subdivision of the zonal soil order comprising a large group of soils in which sesquioxides increase relative to silica during soil formation. Pedalfers usually occur in humid areas...

s and pedocal

Pedocal

Pedocal is a subdivision of the zonal soil order. It is a class of soil which forms in semiarid and arid regions. It is rich in calcium carbonate and has low soil organic matter. With only a thin A horizon , and intermittent precipitation calcite, other soluble minerals ordinarily removed by water...

s are subdivisions within the zonal soil order. Examples of zonal soils are the latosol

Latosol

Latosol is a name given to soils found under tropical rainforests. They are red or yellowish-red in colour throughout and they do not have distinct horizons like a podsol. The red colour comes from the oxides of iron and aluminium which remain in the soil...

 or the podsol

Podsol

In soil science, podzols are the typical soils of coniferous, or boreal forests. They are also the typical soils of eucalypt forests and heathlands in southern Australia...

 (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

Parent material

In soil science, parent material is the underlying geological material in which soil horizons form...

 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

Calcareous

Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...

 soils develop from a limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

. It has two sub-types:

Rendzina

Rendzina

Rendzina is a dark, grayish-brown, humus-rich, intrazonal soil. It is one of the soils most closely associated with the bedrock type and an example of initial stages of soil development...

 soils are thin soils with limited available water capacity

Available water capacity

Available water capacity or available water content is the range of available water that can be stored in soil and be available for growing crops....

.

Terra Rossa soilss are deep red soils associated with higher rainfall than Rendzina.

Hydromorphic soils form in wetland

Wetland

A wetland is an area of land whose soil is saturated with water either permanently or seasonally. Wetlands are categorised by their characteristic vegetation, which is adapted to these unique soil conditions....

 conditions. There are two sub-types:

Gley soil

Gley soil

Gley soil in soil science is a type of hydric soil which exhibits a greenish-blue-grey soil color due to wetland conditions. On exposure to the air, gley colors are transformed to a mottled pattern of reddish, yellow or orange patches. During gley soil formation , the oxygen supply in the soil...

s - 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

Peat

Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...

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

Halomorphic soils form due to soil salination

Soil salination

Soil salinity is the salt content in the soil.- Causes of soil salinity :Salt-affected soils are caused by excess accumulation of salts, typically most pronounced at the soil surface. Salts can be transported to the soil surface by capillary transport from a salt laden water table and then...

.

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.