Agriculture Reference
In-Depth Information
Aggregate formation through abiotic processes becomes more important in soils with
appreciable clay contents which crack along lines of weakness due to the tensile forces
created during drying. Other factors include the pressures associated with drying as roots
absorb water from the soil and the working of soil by man and soil invertebrates. Aggregates
are bound together by cements which may be either organic or inorganic in nature.
Aggregates are classified into size groups that reflect their differing internal compo-
sitions. Particles up to approximately 250 equivalent cylindrical diameter (ECD)
have been termed micro-aggregates and may be involved in dispersion: flocculation
reactions dependent on surface properties (Oades‚ 1987). Macro-aggregates are defined
as those greater than 250 ECD and are bound together by cements such as oxides of
iron‚ aluminium‚ polysaccharides and humic polymers. At a larger scale of organisation‚
a different type of binding occurs through the action of fungal hyphae (notably those of
mycorrhizal fungi‚ Miller and Jastrow‚ 1990; Chenu‚ 1993; Dorioz
et al
.‚ 1993; Foster‚
1994) and small roots (Figure I.19).
In well-structured soils‚ aggregates do not increase in a smooth gradation of sizes.
Large aggregates comprise those of the smaller sizes held together in a hierarchical
arrangement by characteristic suites of binding agents. Tisdall and Oades (1982) have
described the nature of the binding agents operating at different scales within aggregates
from an Australian red-brown earth (alfisol) (Figure I.20). While the processes involved
may result in different size groupings of aggregates in particular soils‚ the principles are
likely to apply to those soils in which organic matter is the main binding agent.
Water-stable aggregates less than 2 ECD include structures in all stages of con-
densation from relatively-open floccules to condensed tactoids. Some may be cemented
with iron and aluminium oxides. Aggregates in this size range may be highly resistant to
dispersion because of particle bonding by the stabilising aromatic and insoluble organic
matter present. These aggregates are produced only slowly under pasture and may
persist in cultivated soils for many years (Oades‚ 1987).
