Agriculture Reference
In-Depth Information
Box 1.4
The Mobility of Elements in Soil
The mobility of an element depends on its solubility in water and the effect
of pH on that solubility. Relative mobilities, represented by the Polynov series in
table B1.4.1, have been established from a comparison of the composition of river
waters with that of igneous rocks in the catchments from which they drain. The
low mobility of Al and Fe is explained by the formation and strong adsorption of
hydroxy-Al and hydroxy-Fe ions at low pH, and the precipitation of insoluble Al
and Fe hydroxides at higher pHs. The least mobile element is titanium, which
forms the insoluble oxide TiO
2
. Therefore, titanium is used as a reference element
to estimate the relative gains or losses of other elements in the profile.
Table B1.4.1.
The Relative Mobilities of Rock Constituents
Constituent elements
or compounds
Relative mobility
a
Al
2
O
3
0.02
Fe
2
O
3
0.04
SiO
2
0.20
K
1.25
Mg
1.30
Na
2.40
Ca
3.00
SO
4
57
a
Expressed relative to chloride taken as 100
in greater leaching of soluble materials, which can be removed from the soil en-
tirely. However, in arid climates where evaporation generally exceeds rainfall, there
is a net upward movement of water most of the time, which carries salts to the
soil surface.
The extent to which salts are retained in the soil profile depends on the mo-
bility of an element (box 1.4) and the rate of drainage through the soil. In arid
areas, even the most soluble constituents, mainly sodium chloride (NaCl) and to
a lesser extent the chlorides, sulfates, and bicarbonates of Ca and Mg, tend to be
retained and give rise to
saline soils
(section 7.2.2). However, in more humid cli-
mates, there is a greater loss of salts and SiO
2
, and the soils are more highly leached,
except in the low-lying parts of the landscape where the drainage waters and salts
accumulate (section 1.3.4).
Lessivage
. Downward movement of water can mechanically wash, or translo-
cate, clay particles from the A to B horizon. In humid temperate climates, on
acidic parent materials, such lessivage is frequently associated with the leaching of
organic compounds from the litter layer. These compounds form complexes with
iron (Fe) and aluminum (Al) in the A horizon. The net result is that minerals,
such as the iron oxides, which color the soil yellow to red, are dissolved from the
A horizon and deposited in the B horizon. The lower part of the A horizon (A2)
becomes pale or “bleached” below the upper part (A1), which is darkened by the
presence of organic matter. Such soils are called “podzolized,” after the Russian
