Agriculture Reference
In-Depth Information
Figure 1.10.
The 2.5YR (yellow-red) page of a Munsell color book. The value is from white at
the top to black at the bottom. Chroma becomes higher from left to right.
other colors do not occur in soil; they do. It is common to find gray soils in
which no hue occurs, and in this case only the value and chroma are indicated
as shown in Figure 1.2, horizon Btg2. Under highly reducing conditions blues
will sometimes be found. Most topics used by soil scientists include a gley page
for such soils. A picture of a page in a Munsell color book commonly used by
soil scientists is shown in Figure 1.10.
Soil color deserves special attention. When rock is ground up, it produces
a gray powder. Gray occurs in E and gleyed horizons under acid and reduc-
ing conditions, and thus this color provides important information when ana-
lyzing such as soil. The normal soil colors red and black are derived from iron
and organic matter, respectively. Iron is in the form of oxides and hydroxy
oxides (Fe
2
O
3
, FeOOH, etc.). Black and brown colors are normally derived
from highly decomposed organic matter and humus.
As a soil develops, decomposition of organic matter (OM) occurs, produc-
ing humus, which is black; and iron is released from minerals by weathering,
which yields various reds and yellows; both mechanisms yield soil coloring
agents. Under oxidizing conditions, where soil is not saturated with water, the
iron will be oxidized and thus in the ferric state [Fe(III)]. When the iron and
organic matter are deposited on the surfaces of sand, silt, clay, and peds, they
develop a coat that gives them a surface color. However, soil color is not only
a surface characteristic but extends through the soil matrix. Under oxidizing
conditions soil has a reddish color. The chroma of this color depends to some
extent on the amount of and the particular iron oxide present.
