Agriculture Reference
In-Depth Information
CHAPTER
3
SOIL BASICS III
T
HE
B
IOLOGICAL AND
O
RGANIC
C
OMPONENTS IN
S
OIL
It might be assumed that soil is basically a dead, inert, nonliving material,
which it is not. A more accurate view is that soil is living or alive. It has living
organisms, which appear to be mostly plants, in and in intimate contact with
it. Plants have two parts, which interact with soil the tops and the roots. Both
are important, although roots have a more intimate and immediate effect on
the biological, physical, and chemical properties of soil. Many other types of
organisms (i.e., microorganisms and animals), both large and small, live in soil.
Microorganisms come in a wide variety of sizes, shapes, and abilities in terms
of the types of environments they live in and the reactions they carry out. It
is common knowledge that worms and prairie dogs live in soil, but these serve
only as representatives of the variety of animals present.
Animals and plants, especially roots and microorganisms, provide bio-
chemical, bioorganic, and organic compounds in soil. These may be in the
form of cellular components, such as cell walls and membranes, enzymes, and
complex and simple organic compounds. Decomposition of complex cellular
material and biochemicals leads to the formation of simpler intermediate
bioorganic and organic compounds. Aerobic decomposition of any organic
matter in soil eventually leads to the production of carbon dioxide and water,
anaerobic decomposition leads to the same products plus methane, and both
result in the synthesis of humus, which is a dark-colored, high-molecular-
weight, and highly complex material. The overall reaction of organic matter in
soil is illustrated in Figure 3.1.
Organic compounds interact with each other and with the inorganic, macro,
micro, and colloidal components in soil. Complex and simple organic mole-
cules form complexes with inorganic cations and anions and with colloidal
carbon and clay in the soil solution. Some enzyme-soil complexes may have
increased catalytic activity, while others have decreased activity or are com-
pletely inactive. Also, some combinations and complexes may render organic
matter resistant to decomposition. Eventually organic matter and complexes
may become stabilized such that they have long residence times in soil.
All of these factors are active in all soil at all times and may affect any
analysis. Simpler organic and inorganic molecules, compounds, and ions may
