Geoscience Reference
In-Depth Information
Table 15.2.
Soil orders of North America. (Modified from Soil Survey Staff,
2010
.)
Soil Order
Characteristics
Alfisols
Arable soils with
>
3 consecutive months with enough soil water for plant
growth
Andisols
Soils formed in volcanic ash, with abundant volcanic glass (Andosols)
Aridisols
Dry, desert-like soils, often rich in calcium carbonate, with low organic content
and sparse vegetation cover
Entisols
Soil with little or no profile development and lacking diagnostic soil horizons
except for weak A-horizon (Arenosols; Fluvisols; Regosols)
Gelisols
Cold-climate soil with permafrost within 2 m of the surface
Histosols
Highly organic soil with at least 20-30 per cent organic matter by weight in a
layer at least 40 cm thick (Histosols)
Inceptisols
Soil with weakly developed horizons, depleted of bases and/or iron and
aluminium but with some weatherable minerals; occurs mainly in humid
and subhumid areas
Mollisols
Soils formed under grassland in semi-arid to subhumid areas; rich in humus,
bases and calcium carbonate (Chernozems; Kastanozems)
Oxisols
Thick weathered soils of the humid tropics; mostly depleted of unweathered
minerals; red to yellow colours (Ferralsols)
Spodosols
Ashy grey acidic soils formed on sands; strongly leached surface layer;
subsurface accumulation of humus mixed with amorphous iron and/or
aluminium (Podzols)
Ultisols
Weathered red/yellow clay-rich acidic soils low in bases (Ferralsols)
Vertisols
Heavy, dark churning clay soils with deep vertical cracking in the dry season;
contain abundant swelling clay minerals (notably, montmorillonite);
variable salinity and alkalinity (Vertisols)
15.3 Soil classification
During the late nineteenth and early twentieth centuries, the great pioneering soil
scientists Dokuchaev, Marbut and their co-workers mapped the soils of the drier
regions of Russia and North America at a broad reconnaissance level. In Russia, the
soils were aligned roughly parallel to latitude, giving rise to the notion that climate
was the primary control over soil distribution, with podzols in the colder north and
chernozems in the warmer, somewhat wetter south. North America is aligned in a
different way, with the Rockies separating west from east and producing a pronounced
rain-shadow effect in the Great Basin. Early soil mapping defined a zone of pedalfers
in the more humid east and pedocals in the drier west, with pedalfers being soils rich
in aluminium and/or iron and pedocals being soils rich in calcium carbonate. Since
that time, soil classification has greatly progressed, and in North America, twelve Soil
Orders have now been recognised (Soil Survey Staff,
2010
and
Table 15.2
).
Efforts to secure an international classification began with the FAO-UNESCO Soil
Map of theWorld at 1 in 25million scale and its successive versions published between
1971 and 1981. From 1982 to 1991, this task was delegated to the International
