Geoscience Reference
In-Depth Information
is sometimes not easily recognized. Their name is related to the Latin
lixivium
meaning solution, lye. The relatively rich illuvial B horizon contains low-activity
clays with a moderate to high base saturation. Since kaolinites and halloysites pre-
vail, the cation exchange capacity is low in the entire soil profi le. The subtropical to
tropical climates of these soils differ from the climates of the majority of other ABC
soils owing to winters being dry while summers being humid. Lixisols belong to old
soils that started to form earlier than the Pleistocene more than 2 million years ago
during conditions much more humid than those of today. Their reddish to yellow
color is the result of dehydration of ferrihydrite to hematite (rubefaction) due to the
existence of prolonged dry seasons. Although Lixisols have low levels of readily
available plant nutrients, owing to their high content of bases, they lack the Al toxic-
ity manifested by Ferralsols and Acrisols. They are well drained and their water-
holding capacity is better than that of Ferralsols and Acrisols. They are also classifi ed
in national taxonomies as Red-Yellow Podzolic Soils, Argissolos, Sols Ferrallitiques
Faiblement Desaturés Appauvris, Red and Yellow Earths, and Latosols. They form
subgroups of Alfi sols in the US Taxonomy.
10. The last set is reserved for relatively young soils or soils with very little or no
profi le development. They are grouped together with very homogeneous sands:
Umbrisols
,
Arenosols
,
Cambisols
, and
Regosols
.
Umbrisols are soils without a distinct profi le development or with a relatively
fresh soil with a great accumulation of organic material only partly humifi ed,
described frequently as raw humus or as mor, moder, and mull. Their name is related
to the Latin
umbra
meaning shade. This organic A horizon is not mixed with min-
eral portion of soil, mainly sand in the C horizon on the bottom of the soil profi le.
Due to the combination of acidity, low temperature, and excess surface wetness,
there is a slow biological transformation of plant remnants. Since Umbrisols were
never cold or wet enough to have developed a distinct humus horizon of Histosols,
they are classifi ed as a separate RSG. Some national systems describe them as
Sombric Brunisols and Humic Regosols and Brown Podzolic Soils. The US
Taxonomy ranks them in lower taxons in Great Groups of Entisols and Inceptisols.
Arenosols are various forms of sands that originated in place either by climatic
weathering of quartz-rich sediments, rocks, and wind-deposited sands or by earlier
seawater weathering of shallow sea shelves. Their name is derived from the Latin
harena
meaning sand and from the similar French
arene
or Spanish
arena
. The top
humus A horizon is either underdeveloped or entirely missing. Although their exis-
tence does not depend upon the climate, properties and types of eventual agricul-
tural use of Arenosols differ according to their phase of soil development that
increases within climatic zones from arid to subhumid temperate to humid tropics.
Owing to a lack of clay particles, all Arenosols existing in one of these three cli-
matic zones have common physical characteristics of very high permeability, very
low water retention (fi eld capacity), no aggregation, very low cation exchange
capacity (CEC), low fertility, and unfavorable hydrologic conditions for plants -
even those that are drought resistant. Moreover, whenever thin hydrophobic fi lms
occasionally cover the sand particles, the hydrologic soil properties of infi ltration
