Chemistry Reference
In-Depth Information
horizons. The
B horizon
is the layer of intermediate weathering in which
small particles, washed down from the
A
horizon, are mixed with reprecip-
itated minerals initially leached out also from the
A
horizon. The
B
horizon
has some distinctive characteristics: it generally is of a color different from
that of the
A
horizon, is more intensely red or brown than the
A
horizon,
and often includes accumulations of clay and/or organic matter or a com-
bination of these two. In most temperate climates, the
A
and
B
horizons
together make up about one meter in depth. The lowermost
C horizon
is com-
posed largely of mineral fragments and rock particles; it is little affected by
soil-forming processes and does not share the properties typical of the over-
lying horizons.
Composition and Properties of Soils
Over 98% of the soil is made up of only eight major chemical elements, listed
in Table 51, in order of decreasing abundance. The 90-odd others make up
the remaining 2%; many occur in the soil as secondary or minor elements,
while a large number of still others are present in only very low, often trace,
concentrations. Thus all soils contain main, minor, and trace elements com-
bined into chemical compounds and aggregated into complex particles of
varying shape, size, and chemical composition (see Textbox 8).
The size, and particularly the size distribution of the particles (or PSD,
as
particle size distribution
is often abbreviated), which is closely related to the
texture of the soil, is an important characteristic that determines many of the
TABLE 51
Elemental Composition of Soil
Element
Average abundance (wt%)
Oxygen
47
Silicon
27
Aluminum
7
Carbon
5
Iron
5
Calcium
3
Sodium
2
Magnesium
2
Potassium
1
Other elements
a
below 1
a
Totaling 84, these elements include, in descending order of
abundance, titanium, hydrogen, phosphorus, nitrogen, barium,
and strontium, each one of them in less than one percent.
