Agriculture Reference
In-Depth Information
and chemically. Physically they are mostly colloidal in size and thus remain
suspended in water and have large surface areas (unit mass basis). There are
a large and varied number of clays that differ in terms of both the arrange-
ment of their components and in the components they contain. Here we will
consider four common types of clays found in soil that are models for all
soil clays. These clays can be grouped into two classes: (1) clays composed of
layers of alumina octahedral sheets and silica tetrahedral sheets and (2) the
amorphous clays.
There are three potential sources of soil clays:
1. Clay that is present in rock and is released when rock breaks down as a
result of physical processes. These clay structures are subject to change
once released from rock.
2. Clays released by chemical breakdown of rock, which can lead not only
to the release but also the formation of clays.
3. Clays formed in soil by chemical reactions occurring after rock
decomposition.
Soil components silica and alumina are solubilized, in low concentration, and
can react, or crystallize, to form new clays. In addition, clays from any source
change over time and become simpler and simpler. Silica is more soluble than
alumina and so the silica : alumina ratio decreases over time. Eventually this
leads to deposits of alumina that are used to as an aluminum ore for the pro-
duction of aluminum metal. Although these reactions are considered to be
very slow on a human timescale, they do occur.
Clays composed of layers are called
layered silicates
. The most common
sheets are of silicon tetrahedra and aluminum octahedral (see Figure 2.2).
Three common representative clays in soil are the 1 : 1 kaolinite, 2 : 1 fine-
grained micas, and 2 : 1 smectites; that is, kaolinites have one sheet of silicon
tetrahedra and one sheet of aluminum octahedra. The fine-grained mica and
smectites have two sheets of silicon tetrahedral and one sheet of aluminum
octahedra. These latter clays have a sandwichlike arrangement with the alu-
minia octahedral sandwiched between two silica tetrahedral layers.
In terms of soil development and the development of soil horizons, the
smectites and fine-grained micas are found in younger, less weathered soils.
Kaolinite is found in highly weathered soils. Considering a time sequence, at
the beginning of formation soil will contain more complex clays that weather
to simpler forms over time. However, it is convenient to start with a descrip-
tion of the simpler layer silicate clays and then describe the more complex
clays.
An additional important characteristic of clays is their surfaces, which are
distinguished as being either external or internal. Internal surfaces occur, for
example, in nonexpanding 2 : 1 clays, such as the fine grained micas, and are
generally not available for adsorption, chemical, or exchange reactions.
