Geoscience Reference
In-Depth Information
gravel content, they have high hydraulic conductivity values and manifest rapid
internal drainage. As a result, they are sensitive to drought during rainless periods
even in humid climate. Although they are suitable for grazing, they are much more
benefi cial to forestry. The national systems recognize them as Petrozems and
Litozems or as subtypes Leptic Rudosols and Tenosols. If they are on calcareous
rocks, they belong to subtypes of Rendzinas that earlier belonged to the principal
soil types owing primarily to their neutral or slightly alkaline reaction (pH) thus
forming a positive medium on which to plant the great majority of agricultural
plants. Even now some national systems keep them on the top classifi cation level
using a variation on the original term like Eurendzina. The US Taxonomy recog-
nizes Leptosols as Lithic Entisols at a lower taxonomic level. Rendzina soils belong
therefore within the order Entisols to great groups.
4. Soils strongly infl uenced by water. The infl uence varies in many different
aspects. There are alternating wet and dry conditions that highlight the important
role of swelling and shrinkage. There are regularly fl ooded soils. There are soils
infl uenced by fl uctuating high groundwater level. There are soils with high con-
centrations of dissolved salts transported by the fl ux of soil water. As a result, we
recognize the following RSGs:
Vertisols
,
Fluvisols
,
Gleysols
,
Solonchak
, and
Solonetz
.
Vertisols denote heavy clay soils containing a high portion of swelling clay min-
erals of smectite type whose properties also depend upon the value of exchangeable
sodium percentage (ESP). During regularly occurring long-time rainless seasons,
they dry out, and as they shrink, deep cracks appear and are subsequently fi lled by
dust from neighboring soil surfaces. With the onset of rain, each individual area of
heavy clay surrounded by cracks tends to swell, but the original cracks now fi lled by
“foreign” soil material do not swell. With each small area of clay continuing to
swell upward to a shape akin to a loaf of bread, the microrelief across the entire soil
surface forms a shape of an ensemble of many loaves within the originally formed
cracks. Such a microrelief is called gilgai. All together there exists a constant inter-
nal soil turnover, which gave the RSG name derived from the Latin
vertere
which
means to turn. Alternate shrinking and swelling causes self-mulching, where the
soil material consistently mixes itself, causing Vertisols to have an extremely deep
A horizon. Vertisols are most frequent in semiarid tropics with average annual rain-
fall above 500 mm. They have high agricultural potential provided that they are
irrigated and that the planted crop roots are not damaged by volumetric changes.
Since they occur on large plains, they are suitable for large-scale mechanization and
irrigation systems. However, tillage is hindered by the stickiness in wet soils and by
hardness and large clods in dry soils. Infi ltration into a dry soil with well-developed
fi ssures is initially very high when water fl ows easily and quickly into large cracks,
but after the cracks are fi lled, the infi ltration through the clay surface is very slow.
Owing to their soil hydrologic restrictions, the range of readily available water
between wilting point and fi eld capacity is small. Some of the local names which
penetrated into several national classifi cation systems are Black Cotton Soils, Black
Turf Soils, Regur, Margalites, and Vlei Soils or in modifi ed forms like Vertisols and
Vertissolos. The US Taxonomy recognizes the order Vertisols.
