Introduction to Soil Chemistry

62 soil basics iii
H H
O
H
O
H
H
H H
+ H+
H +
M +
M +
H +
O
O
H
H
H
H
H
H
O
H
O
H
O
H
O
H
O
H
O
H H
O
O
M +
O
C
M +
H +
H H
O
H H
O
O H
H
H
H H
O
O
O
H
O
O
C
H
H
C
H
H
H
O
H +
O
H
C C
a source of simple organic molecules during its decomposition. These compounds
help determine the microorganisms present in the plant’s rhizosphere.
No matter what the source of organic matter or the mechanism of its
decomposition, an extremely important compound, humus, is formed during
decomposition [6–10,17].
3.6.1. Humus
Humus is the material synthesized or resynthesized during decomposition of
organic matter by microorganisms. It is produced under both aerobic and
anaerobic conditions and remains after decomposition of the original organic
matter is complete. Humus is a complex molecule often described as being a
polymer even though no mer 3 unit has ever been found. It is black or dark
brown in color and has a high affinity for organic molecules, cations, and water.
Organic molecules associate with humus via a process that is similar to the
molecule dissolving in it. Cations are held by cation exchange involving both
acid and phenolic groups (see Figures 3.10 and 3.11). A base extract of humus
O
H
Humus
M +
++
M +
H
O
H H
O
C C
H
O
Figure 3.10. Humus with cation exchange sites created by the ionization of phenolic and acidic
functional groups. The M + represent exchangeable cations.
3
A mer is the individual repeating unit from which a polymer is formed. Thus polyethylene is
made of ethylene units, the mer, bonded together.
O
–
H +
H
O
H