Geoscience Reference
In-Depth Information
left as traces of the earlier accumulation of salts. The permanent “surviving” of Na
+
and Mg
2
was enabled by their fi xation on exchangeable positions of the solid parti-
cles, mainly smectites. Solonetz have evolved on unconsolidated materials of fl at
lands having a climate typifi ed by long hot rainless summers. Annual precipitations
are higher than those linked with Solonchak, but still below 500 mm. If Solonetz
evolve from Solonchak during long-term irrigation, then the main factor infl uencing
the process is high content of sodium salts either in earlier Solonchak parent mate-
rial or due to the use of Na
+
-rich irrigation water. The soil profi le refl ects the domi-
nant process of washing out the soluble salts. Due to the decreased salt concentration
and prevalence of exchangeable Na
+
(partly also Mg
2+
), the clay particles cannot
coagulate and stick together. Inasmuch as they exist in a peptized state of individual
colloidal units having diameters well below 1
m, they are simply transported out
of the top horizons through slightly bigger sized pores. This eluviation of tiny col-
loids is the dominant mechanism for a thin natric horizon to develop below a black
or gray brown surface humus horizon. Its gray whitish color, its high pH value
(about 8.5 or even more), and its columnar aggregates with rounded tops are typical
properties of natric horizons. Signs of gleization could appear at the bottom of nat-
ric horizons. When wet, they are dispersed and sticky and have very low hydraulic
conductivity and low air permeability. When they are dried, they form a hard crust
inside their soil profi le and wide cracks on their surface. Solonetz could be brought
into agricultural use only after an expensive amelioration that starts with planting
special Na-resistant grass and incorporating gypsum or calcium chloride in the top-
soil. Next, gradual deepening soil tillage mixes the ameliorated topsoil with the
upper part of the natric horizon. As Ca
2+
cations replace Na
+
in their exchange posi-
tions, some coagulation begins and the fi rst step of aggregation takes place. The
amelioration is expensive and requires an extremely large number of years to obtain
only partially acceptable results. Hence, it is not surprising that the vast majority of
Solonetz have not been reclaimed - they are either used for extensive grazing or left
lying fallow. The old classifi cations denoted them as Alkali Soils, Sols Sodiques, or
Schwarzalkaliböden. In the US Taxonomy, Solonetz corresponds to sodium-rich
Aridisols
and
Mollisols
.
ʼ
5. The fi fth set of soil groups is the RSGs in which iron (Fe) and/or aluminum (Al)
chemistry plays a major role in their formation:
Andosols
,
Podzols
,
Plinthosols
,
Nitisols
, and
Ferralsols
.
Andosols developed mainly on volcanic ashes and glasses but also on tuff, pum-
ice, cinders, and other volcanic ejecta in the circum-Pacifi c zone (New Zealand, the
Philippines, Japan, Ecuador, Peru, etc.). They also occur in humid climate on other
silicate-rich materials. Rapid weathering of porous materials results in stable com-
plexes of minerals with humus. The top A horizon is dark to black, with fi ne struc-
ture. The color gave the name to those soils, stemming from Japanese
an
meaning
black and
do
meaning soil. The clay fraction is composed mainly of allophane, the
major weathering product of volcanic materials with an admixture of halloysite.
This composition predetermines the properties of the A horizon. The soil is very
porous mainly due to the spherical shape of allophane. It is well aggregated with
