Environmental Engineering Reference
In-Depth Information
Figure 18.11
The formation of humus by the decomposition
of plant constituents and the synthesis of microbial products.
their death. SOM is not simply a residual product, after the soil organisms have attacked
and partially decomposed the dead plant structures. Much of it consists of freshly made
microbial products which are very influential in giving humus its important properties.
Figure 18.11 illustrates how an active organic cycle can rapidly break down large
quantities of organic matter by the activities of bacteria and fungi. Larger organisms such
as earthworms act to mix the organic matter throughout the topsoil. The end result is a
relatively stable mild humus or mull (pH >5·5).
If the organic cycle is less active, with slow decomposition and the build-up of only
partially fermented litter, a very different SOM results. Acid soil conditions result from
an acid parent material, or excessive leaching or acid-tolerant vegetation. The population
of bacteria is reduced, and slow decomposition by fungi will produce a raw humus or
mor
. This is usually layered into litter (L), a fermentation (F) layer and a very thin humus
(H) layer. Mor is not well decomposed, or mixed into the soil profile, in the absence of
both bacteria and earthworms. An intermediate stage (pH 4·5-5.5) produces
moder
humus. Figure 18.12 shows the three types of SOM which are found in well drained
natural soils. The nature of the soil, climate and vegetation will determine which of the
three types is formed. In poorly drained, waterlogged situations the lack of oxygen
excludes many fauna and soil micro-organisms. Decomposition is very slow, and the
remains of plants build up into a
peat
.
