Agriculture Reference
In-Depth Information
Pore space
Domains
1 m
Quartz grain
Diagram of a microaggregate
(White 1997). Reproduced with
permission of Blackwell Science
Figure 3.3
Organic polymers
Bacteria
Ltd.
saccharides and clay surfaces, are also important. Inorganic compounds act as in-
terparticle cements and stabilization agents. Examples of cementing agents are (1)
CaCO 3 in calcareous soils, (2) sesquioxides as films between clay crystals and as
discrete charged particles in many acid, highly weathered soils, and (3)SiO 2 at
depth in highly weathered profiles, where a hardened or indurated layer may form.
Microaggregate stabilization is relatively insensitive to changes in soil man-
agement, except when management induces marked changes in the dominant
cations on clays (e.g., Na replacing Ca 2 ), or changes in soil pH (acid alka-
line pH decreases the positive charges on kaolinite edge faces and sesquioxide sur-
faces), or physical disruption of aggregates by repeated cultivation that exposes
“protected” organic matter to additional decomposition by soil organisms.
Macroaggregates . Figure 3.4 shows a model of a macroaggregate. The forma-
tion of a macroaggregate depends mainly on the stable cohesion of microaggre-
gates in the larger structural unit. This stabilization is much more dependent on
management than is stabilization of the microaggregates themselves, because it is
achieved primarily through plant roots and fungal hyphae. Roots can penetrate
pores 10
m. In so doing,
they enmesh soil particles to form stable macroaggregates. Grass swards are espe-
cially effective because of the high density of fibrous roots in the surface soil.
Deposits of C compounds in the rhizosphere (section 2.3.1) provide the sub-
strate for microorganisms that produce polysaccharide gums and bind soil parti-
cles together. These effects all depend on maintaining a high level of biotic ac-
tivity—plants, animals (mainly earthworms), and microorganisms—in the soil.
This activity is most important in soils of pH 5.5-7, which derive little benefit
through aggregate stabilization by either sesquioxides or CaCO 3 . Overall, macroag-
m diameter, and fungal hyphae enter pores 1
 
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