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
A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...
characteristics that reflect the dominant influence of some resident factor of relief or
parent material
Parent material is the underlying geological material (generally bedrock or a superficial deposits, superficial or drift (geology), drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from th ...
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
Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines.
In zoology
''Calcare ...
soils develop from a
limestone
Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
. It has two sub-types:
::
Rendzina soils are thin soils with limited
available water capacity Available water capacity is the amount of water that can be stored in a soil profile and be available for growing crops. It is also known as available water content (AWC), profile available water (PAW) or total available water (TAW).
The concept, p ...
.
::Terra rossa soils are deep red soils associated with higher rainfall than rendzina.
:Hydromorphic soils form in
wetland
A wetland is a distinct semi-aquatic ecosystem whose groundcovers are flooded or saturated in water, either permanently, for years or decades, or only seasonally. Flooding results in oxygen-poor ( anoxic) processes taking place, especially ...
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
Peat is an accumulation of partially Decomposition, decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, Moorland, moors, or muskegs. ''Sphagnum'' moss, also called peat moss, is one of the most ...
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
Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order ( anisotropy) within soils. These alteration ...
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
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{{soil type
Soil in the United States
Taxonomy
image:Hierarchical clustering diagram.png, 280px, Generalized scheme of taxonomy
Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme o ...
Usda Soil Taxonomy, 1938
Soil science