Geoscience Reference
In-Depth Information
rate and water-holding capacity are adversely modifi ed. In the transition zone
between typical Sahara desert soils and Sahelian soils, the unfavorable properties of
Arenosols are gradually diminishing, but not disappearing. In national systems we
fi nd Arenosols as Classe Des Sols Peu Évolués, Sols Minéraux Bruts, Red and
Yellow Sands, Arenic Rudosols, and Psamnozems. The US Taxonomy recognizes
them either as suborders of Entisols (Psamments) or as subgroup in Alfi sols.
Cambisols are mainly soils in the early state of differentiating the soil profi le into
horizons or soils that are in the stage of transition from AC profi le into ABC profi le.
The B horizon shows the alteration relative to the lower C horizon in color and the
removal of calcium carbonate when the C horizon is formed by loess. However,
CaCO
3
could be found in B horizon, but in lower concentration than it is in C hori-
zon in some instances. It keeps the neutral or slight acid reaction. Texturally their
content of clay particles is higher than in sandy loam. The structure in B horizon is
usually polyhedral and differs from the crumbling structure of the top A horizon.
Hydrolysis of iron-containing minerals like biotites, pyroxenes, and amphiboles
produces ferrous iron that is partly oxidized to goethite and hematite and partly
exists as a free iron fi lm covering the sand and silt particles. These “coats” give the
distinct brown color to B horizon in well-developed Cambisols. There is no leach-
ing of bases and sesquioxides. The name was derived from the Latin
cambiare
which means to change. They are fertile soils used from the start of the agricultural
revolution in the mild zone. Many Cambisols belong to the most productive soils of
the earth. The old classifi cations and some contemporary national systems recog-
nize them as Brown Soils, Braunerde, Sols Bruns, and Brunizems due to the brown
color of their B horizon. The US Taxonomy classifi es them in Inceptisols.
Regosols are very weakly developed soils originating on unconsolidated mineral
parent material. They lack distinct horizons. If we describe the thin top layer differ-
ing from the parent material, we use the term ochric horizon. Its name is derived
from the Greek
ochros
which means pale. It has very low content of organic matter,
the weathering is usually weak, or in some instances their products form a hard
crust. Regosols have a very low capacity to retain water, are sensitive to droughts,
and are easily eroded. With a low nutrient content sometimes approaching zero
coupled with poor hydrologic relations, their agricultural signifi cance remains vir-
tually nil. Regosols are classifi ed in national systems as Skeletal Soils, Rohböden,
and Sols Peu Évolué. The US Taxonomy recognizes them as lower units (great
groups) of Entisols.
13.2
USDA Soil Taxonomy
The fi rst US soil classifi cation system at the end of the nineteenth century was based
on the underlying geology. Soil maps elaborated for agricultural administration and
supposedly for the benefi t of all farmers were little more than maps of geologic
forms appearing on the landscape surface combined with an empirical guess regard-
ing soil texture. A principal modifi cation eventually occurred in the 1920s when
