Agriculture Reference
In-Depth Information
biomass (especially coarse roots) than grasslands and, for a given type of ecosystem,
the ratio of above-ground to below-ground biomass tends to be higher in tropical than
temperate regions.
3.2.4
SOIL ORGANIC MATTER
3.2.4.1
Quality
In mineral soils, organic matter comprises only a small proportion of the total soil volume.
In terms of its utilisation, a major factor is the size of the organisms exploiting it.
Soil organic matter has a markedly discontinuous distribution at the micro-organism
scale while, at the larger scale of geophagous invertebrates, it is effectively continuous,
although of low energy density. A sample from the upper 10 cm of a savanna soil with
ca.
1 % organic matter in West Africa had an energy value of (Lavelle, 1978)
while, in an English beech (
Fagus sylvatica
) wood, a soil with 18 % organic matter had
an energy value of (Bolton and Phillipson, 1976).
The quality of soil organic matter, when treated as a resource, depends on its:
chemical properties (as defined in section I.3.2.2.1);
the shape and proportional representation of different particle size classes (
(i)
(ii)
the par-
ticle-size distribution); and
(iii) accessibility in the soil matrix (which, in turn, depends on its inclusion within
aggregates and whether it is combined with minerals to form organo-mineral complexes).
Particle size distribution
Soil organic matter is a mixture of plant and animal residues normally present in all
stages of degradation and comprises particles of a wide range of size classes. Part com-
prises amorphous colloids flocculated following the precipitation of soluble compounds
leached from the surface litter. The larger particles (
i.e.,
those greater than are
assumed to be more susceptible to decomposition than those of smaller size, partly
because the latter have already undergone a degree of microbial degradation.
These smaller particles contain high concentrations of such complex molecules as
lignin and humic materials and are therefore more resistant to microbial attack; such
particles are considered to be chemically protected. Other forms of chemical protection
include the formation of clay:humus and clay:metal ion:humus complexes. Such effects
are greatest with allophane and progressively lower in smectites, illite and kaolinite
(van Breemen and Feijtel, 1990).
Tiessen
et al.
(1984) have confirmed the relationship between soil organic matter
quality and particle size (Table I.29). Carbon and nitrogen concentrations were shown to
increase with decreasing particle size down to while the C:N and C:P ratios
decreased indicating a greater degree of humification of the organic material. This was
associated with increased concentrations of highly aromatic humic acids which further
contribute to their recalcitrance to decomposition, Notably, the finest fraction
differs from the other fine fractions in that it has higher concentrations of fulvic acids
(predominantly aliphatic) and hydrolysable nitrogen (Anderson
et al.,
1981; Tiessen
et
al.,
1984; Catroux and Schnitzer, 1987).
