Agriculture Reference
In-Depth Information
In both undisturbed and agricultural soils‚ water stable macro-aggregates in soils with
adequate carbon concentrations are held together by a fine‚ three-dimensional network
of small roots and hyphae of mycorrhizal and saprophytic fungi and other organisms.
Miller and Jastrow (1990) consider that much of the effect of roots on aggregate formation
is due to the indirect effects of associations with mycorrhizal fungi and that the strength
and nature of this association also depend on the morphology of the root system.
In contrast‚ ephemeral macro-aggregates bonded by transient binding agents are found
in soils of lower organic matter content. Other binding agents‚ such as iron and aluminium
oxides and highly disordered silicate clay minerals may also be present.
Microbial biomass carbon‚ nitrogen and phosphorus concentrations‚ and probably
those of other elements‚ differ between aggregates of different size groups. In comparing
aggregates from forest‚ savanna and cropped soils in a seasonally-dry tropical environment‚
Singh and Singh (1995) found that microbial biomass carbon concentrations were higher
in macro-aggregates (more than 0.3 mm ECD) while micro-aggregates had greater con-
centrations of microbial biomass nitrogen and phosphorus.
The faeces of soil animals may be deposited within soils as void infillings‚ in
remodelled form as linings to galleries and other structures‚ and as discrete aggregates.
Such aggregates occur in a variety of shapes‚ sizes and mineral contents and may
comprise a notable feature of the soil structure‚ particularly in undisturbed or little-
disturbed soils. Aggregates produced by soil animals are therefore highly variable in
composition since their genesis is dependent on the feeding habits of the species that
produced them. The casts of soil-feeding earthworms are perhaps the most well-
recognised form of faunal aggregate. Schemes exist for their classification in soil
micromorphological studies (Bal‚ 1973).
1.3.3.2
Organo-mineral bonding
As indicated above‚ the intimate associations that occur between the various types of
organic matter and the mineral fractions of the soil result from a range of chemical and
physico-chemical bonding mechanisms (Emerson et al .‚ 1986; Theng‚ 1987). Because of
their small sizes and elevated surface areas‚ interactions between clays and organic
matter assume great significance in structural stability (Theng‚ 1987); important
interactions also occur with larger particles (Turchenek and Oades‚ 1979; Chenu‚ 1993).
The most important organic materials in terms of soil aggregation are the polysac-
charides and humic materials‚ although they act at different temporal and spatial scales.
Monnier (1965) categorised the differing effects on soil structure of incorporating
organic materials of varying degrees of humification into the soil. The incorporation of
green materials leads to a flush of microbial activity and the production of such effective
but ephemeral bonding agents as extra-cellular polysaccharides from the organisms
involved (Chenu‚ 1993). If more highly decomposed materials are incorporated‚
the effect is lesser but more long-lasting. Finally‚ if highly decomposed materials rich in
humic compounds are incorporated‚ an even smaller but very long-lasting stabilising
effect is obtained. In the spatial sense‚ the larger aggregates are stabilised by organic
matter derived from crop residues and other regularly-supplied plant materials both of
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