Environmental Engineering Reference
In-Depth Information
The Seventh Approximation (New Soil Taxonomy)
General
In 1960, the USDA published a new classification system referred to as The Seventh
Approximation (SCS, 1960), now called the New Soil Taxonomy. All the soils of the earth
were divided into ten major categories called orders, which were based on distinguishing
characteristics rather than climatic factors. In 1975, two new soil orders were added
(USDA, 1975). The detailed descriptive nomenclature is extremely complex.
Orders
The 12 soil orders are described in brief in Table 7.16. Their worldwide distribution
is given in
Figure 7.114,
and a diagram illustrating the general relationship between
The Great Soil Groups and the soils of The Seventh Approximation is given in
Figure 7.115.
TABLE 7.16
The Soil Orders of the Seventh Approximation
a
Soil Order
General Features
Alfisols
Develop in humid and subhumid donates, have average annual precipitation of 500-1300 mm.
They are frequently under forest vegetation. Characteristic features: clay accumulation in a Bt
horizon, thick E horizon, available water much of the growing season, slightly to moderately
acid
Andisols
Are soils with over 60% volcanic ejecta (ash, cinder, pumice, and basalt) with bulk densities
below 900 kg/m
2
. Characteristic features: dark A horizon, early-stage secondary minerals (allo-
phane, imogolite, ferrihydrite clays), high adsorption and immobilization of phosphorus, very
high canon exchange capacitity
Aridisols
Exist in dry climates. Charactersitic features: horizons of fame or gypsum accumulation, salty
layers, or A and Bt horizons
Entisols
Have no profile development, except a shallow marginal A horizon Many recent over flood-
plains, volcanic ash deposits, unconsolidated deposits with horizons eroded away, and sands
are Entisols
Gelisols
Are soils in very cold climates that contain permafrost within 2 m of the surface
Histosols
Are organic soils (peat and mocks) consisting of variable depths of accumulated plant remains
m bogs, marshes, and swamps
Inceptisols
They especially in humid regions, have weak to moderated horizon development. Horizon
development have been retarded because of cold climated, waterlogged soils, or lack of tame
for stronger development Characteristic feature: Texture has to be finer than loamy very fine
sand
Mollisols
Are frequently under grassland, but with some broadleaf forest-covered soils. Characteristic
features: deep, dark A horizons, they may have B horizons and lime accumulation
Oxisols
Are excessively weathered, whereas few original minerals are left unweathered. They develop
only in tropical and subtropical climates. Characteristic features: often Oxisols are over 3 m
deep, have low fertility, have dominantely iron and aluminum days, and are acid
Spodosols
Are typically the sandy, leached soils of cold coniferous forests. Characteristic features: O hori-
zons, strongly acid profiles, well-leached E horizons, Bh or Bs horizons of accumulated organic
material plus iron and aluminum oxides
Ultisols
Are extensively weathered soils of tropical and subtropical donates. Characteristic features.
thick A horizon, day accumulation in a Bt, strongly acid
Vertisols
Exist most in temperate to tropical climate with distinct wet and dry seasons. They have a high
content of clays that swell when wetted and show cracks when dry. Characteristic features:
deep self-mixed A horizon, top soil falls into cracks seasonally, gradually mixing the soil to the
depth of the cracking
a
After USDA,
Soil Taxonomy
, U.S. Department of Agriculture Handbook No. 436, 1975.
