Agriculture Reference
In-Depth Information
maximum clay formation. These soils are typically very acidic (here “very” is
used relative to other soils), and there is the occurrence of aluminum as Al
3+
.
This is of note because aluminum in this form is toxic to plants. Although there
is a Bt horizon, the clay is the simplest and least active of the clays (see Chapter
2). Spodosols develop in coarse-textured soil under trees, the dentris of which
provides acid to water leaching through the soil profile. Its B horizon contains
illuvial accumulation of highly decomposed organic matter and oxides of iron
and aluminum. In this case the B horizon as shown in Figure 1.4 does not have
an increase in clay.
The pH and clay content of these soils are extremely important in under-
standing their chemistry. The lack of salts, low pH, and dominance of low-
activity clays will greatly affect the retention and extraction of components
from both Ultisols and Spodosols.
In the Aridisol both the occurrence of clay in the lower horizon and the
occurrence of high pH and salt contents will greatly affect the retention of
components. For these soils, analytical procedures must be potentially imper-
vious to high pH levels and salt content, or steps must be taken to remove
salts or change the pH before analysis.
1.6.2.
Soil Color
The color of soil gives an indication of its oxidation-reduction conditions and
the amount of organic matter present. Well-aerated soils will be observed
under oxidizing conditions, and iron will be in the Fe
3+
state, less soluble and
thus less available for chemical reaction. Under water-saturated conditions,
soil will be under reducing conditions as indicated by increased yellow color-
ing, gleying, and mottling. Iron will be in the Fe
2+
state, which is more soluble
and thus more available for chemical reaction. Under these conditions reduced
species such as methane, CH
4
, hydrogen, and other sulfides will be found.
Under saturated or very wet conditions soils tend to have increased
amounts of organic matter. This results in dark colors and dramatically
changes the chemical characteristics of a soil. Organic matter increases a soil's
sorptive and cation exchange capacities and thus alters the movement and
extraction of components present. Organic matter increases ped formation
and stability, thus increasing both aeration and percolation, but under satu-
rated conditions reduction reactions prevail (see Figure 1.9).
While color is often discussed as being closely related to iron and its oxi-
dation state, which in fact it is, it is also related to all other soil components.
When soil color indicates oxidizing conditions, all multiple oxidative state
capable cations are expected to be in their highly oxidized states, and when
soil color indicates reducing conditions, low oxidative states will occur.
1.6.3.
Soil Structure
Increasing soil structure results in increases in both small and large pores,
which means improved water and oxygen movement in and out of soil and
