Agriculture Reference
In-Depth Information
1.1.3.3
Particle size and aggregate fractionation
Organic residues become increasingly fragmented as decomposition proceeds forming
relatively-stable organo-mineral complexes with clay particles. Smaller particles may
comprise the majority of soil carbon. Baldock
et al.
(1992) found that in five diverse
soils including mollisols, andisols and oxisols, the amounts of organic carbon contained
in particles less than 2 mm in size ranged from approximately 80 to 90
%
of the total
amounts present in these soils. Further, with diminishing particle size, an increasing
proportion of this organic carbon is contained within the heavy fraction.
Consequently, smaller particles are often the most highly decomposed and frequently
contain greater amounts of recalcitrant materials, including those derived from micro-
organism activity. These are presumably more resistant to further decomposition than
their larger counterparts. A knowledge of the particle size distribution of the organic
fraction thus allows a broad assessment of resistance to decomposition although this is
less clear in soils such as andisols and mollisols where a high degree of organic matter
protection occurs through the formation of decomposition-resistant organo-mineral
complexes (Duchaufour, 1997; Baldock
et al
., 1992).
The proportion of total soil organic matter present between the aggregates may
be contrasted with that protected from microbial degradation within the aggregates.
Where aggregates are stable in water, these two organic matter fractions may be
separated by sieving the whole soil in water. Total organic matter is assessed following
dispersion of the soil into individual particles through the application of ultra-sound
energy, or by shaking the soil for several hours in water with glass beads. The desired
proportions are obtained by difference (Section I.3.2.4.1) (Feller, 1979; Brucker and
Kilbertus, 1980; Elliott, 1986).
1.1.3.4
Functional characterisation
Organic materials have the following three complementary roles in soils. They:
1. Form a reserve of energy and nutrients;
2. Link mineral components within the soil matrix through their colloidal and charge
properties;
3. Retain cations on their predominantly negatively charged surfaces.
Energy densities and nutrient contents
Litters and soil organic matter have relatively low energy densities. Values for several
fresh litters from different litter systems in eastern France ranged between 18.7 and
20.5 kJ depending on the tree species present (Loustau, 1984) (Table I.8). No signif-
icant changes were observed in the partly-decomposed litters of the F and H layers nor
in the coarse (>50 organic fractions of the soil. Fulvic and humic acids had lower
energy densities (16.7 to 18.8 kJ ) and humin even lower values (8.8 to 12.8), with
the single exception (19.1) of samples from a poorly-drained soil.
Generally, 95 % or more of the nitrogen and sulphur, and between 20 and 75 % of
the phosphorus of surface horizons is found in soil organic matter (Duxbury
et al
., 1989).
Ratios of C:N:S:P appear to be quite uniform with average values of 140:10:1.3:1.3
