Geoscience Reference
In-Depth Information
smectites. They are acidic with limited drainage of the subsoil containing a majority
of smectites in clay. A relatively high concentration of free aluminum and manganese
causes their toxicity for cultural plants. Their profi le consists of a thin humus A
horizon usually poor in organic matter content and with an unstable soil structure.
Below it is a reddish yellow B horizon with the color caused by the illuviated mate-
rial. The parent material is usually rich in active clays like smectites, but their rela-
tion to low-activity clays like kaolinite decreases in individual horizons according
to weathering intensity. As it increases, the concentration of free Al increases
together with accompanying negative properties. If Alisols are used for crop pro-
duction, they need liming to neutralize their acidic reaction and to reduce the solu-
bility of simple inorganic substances containing Al. Nevertheless, they are still
hampered by toxic concentrations of Al and Mn. And even with logical and vigilant
management practices, they remain easily eroded owing to a lack of surface coher-
ence. They exist in national classifi cations as Red-Yellow Podzolic Soils, Fersialsols,
and Sols Fersiallitiques Tres Lessivés. The US Taxonomy recognizes them as
Ultisols.
Acrisols differ from Alisols by their high content of low-activity clay minerals
like kaolinites having a low base saturation. Their reaction is also very acidic. But
similar to Alisols, they have a thin light-colored surface humus A horizon overlying
a whitish to yellow eluviated E horizon with weak structure - sometimes without
any aggregation and strongly massive. It overlies a reddish to strongly yellow illuvi-
ated B horizon. The parent materials are acid rocks and their weathering products.
Acrisols are also poor in plant nutrients and have strongly bound phosphates. Their
acid reaction accounts for their Al toxicity to plants. When covered by forests, their
physical properties contribute positively to regional hydrologic cycles with rain
infi ltration, runoff, and deep drainage entirely adequate, but after being shifted to
agronomic use, their physical conditions deteriorate causing poor infi ltration and
periods of severe soil erosion. They associate with nationally classifi ed soil types
like Red-Yellow Podzolic Soils, Sols Ferrallitiques Fortement Désaturés, and Red
and Yellow Earths. The US Taxonomy recognizes them as suborders in Alfi sols and
Ultisols.
Luvisols are soils of the mild, cool climatic zones north of the Chernozem zone
in the Northern Hemisphere. The average annual precipitations are usually slightly
higher than those of evapotranspiration. Parent materials are unconsolidated sedi-
ments like loess and loessial loams or alluvial and colluvial loams. The humus-rich
top A horizon is slightly leached, and active clay minerals like smectites and illites
are transported by percolating water to the intensive brown illuvial B horizon. Even
if the soil textures of A and B horizons differ substantially with the B horizon more
rich in clay fraction, waterlogging is absent in typical Luvisols. If present, the sub-
type's name refl ects this fact. National and older classifi cation systems used the
terms Pseudopodzolic Soils, Parabraunerde or Braunerde (Brown Soils), Sols
Lessivées, and Gray-Brown Podzolic Soils. The US Taxonomy groups them into
Alfi sols.
Lixisols are strongly weathered soils in which clay was washed out of the eluvial
E horizon to form the illuvial B horizon. Although E horizon is usually very hard, it
