Agriculture Reference
In-Depth Information
1.1.3
THE ORGANIC COMPONENTS
Organic matter accumulates in soil during its formation to reach abundances of 50 to more
than 260 Mg in mineral soils. It is mainly concentrated in the upper 10 to 20 cm
where it comprises several
per cent.
of the total mass of the soil (Sánchez
et al
., 1982;
Post
et al.,
1982).
Almost the whole range of naturally-occurring chemical substances may be found
within soils, since the tissues of all living organisms are returned to the soil on their
death. However, important differences exist in both the physical and chemical properties
and the amounts of the organic materials returned to the soil in different environments
and considerable efforts have been made to characterise soil organic matter in morpho-
logical, chemical and functional terms. Classifications resulting from these approaches
are presented and the properties of the major soil organic components are described,
including their energy density values and physical properties.
1.1.3.1
Morphology and in situ localisation
Soil organic matter occurs in the following forms:
(i) Living macro- and micro-organisms;
(ii) Decomposing animal, plant and micro-organism remains still retaining much of
their original form;
(iii) Exudates and mucus produced by organisms;
(iv) Leachates from above-ground litter and vegetation;
(v) Humic materials.
The living macro-organisms (roots and macro-invertebrates) are readily separated
from soil and are not normally considered to be part of the soil organic matter.
Micro-organisms and the small invertebrates (the micro- and mesofauna) are included in
soil samples and thus comprise that part of the soil organic matter known collectively as
the microbial biomass. Micro-organisms can be readily observed in thin section
preparations of the soil using microscopic and ultramicroscopic techniques. Two major
components routinely noted in these preparations are fungal hyphae and bacteria. Fungal
hyphae ramify through the soil pores often penetrating dead or living plant tissues and
soil aggregates. Bacteria frequently occur as colonies of a few tens of individuals or less
surrounded by polysaccharide capsules on the surfaces of which clay platelets may be
adsorbed (see
e.g
., Figure I.11 A and B).
The faeces of saprophagous invertebrates are a common feature of soil thin sections,
particularly those of surface soil materials (see, for example, Bal, 1982 or Brewer and
Sleeman, 1988). Serial ingestion and egestion of plant structural materials mediated by
a succession of these animals progressively reduces the average size of the decomposing
materials while removing some of the more readily-digested components.
Such decomposing plant materials may retain much of their original structure for
long periods. As decomposition proceeds and the fragments of organic debris are
reduced in size, cell contents are depleted and their walls collapse onto the vacuolar
tannins which have resisted previous decomposition. At later stages, this structure
becomes micro-divided and progressively lysed. Despite this, even in fragments less
