Environmental Engineering Reference
In-Depth Information
Plate 18.1 In semi-arid lands the lack of protection from
vegetation means that raindrop impact breaks down surface
structure, allowing an impermeable crust to form on drying.
Scale in cm.
Photo: Ken Atkinson.
The kinds or shapes of aggregates are summarized in Figure 18.4. Structureless single
grain consists of loose, individual particles as found in raw sands. Structureless massive
consists of a large mass of compacted soil with no recognizable aggregates. It may be
found in wet and raw clays. Crumb is characteristic of soils with mull humus and a very
active soil faunal population, especially earthworms. Blocky , either angular or sub-
angular, is common in many arable topsoils and subsoils in medium-textured soils.
Prismatic is characteristic of subsoils in clay soils affected by shrinking and swelling.
Columnar shows some slaking (dispersion) and is associated with the subsoils of clay
soils with a large sodium content. Both prismatic and columnar form very hard and dense
horizons which are difficult to cultivate with farming implements and which greatly
impede water and root penetration. Platy structures can occur in a variety of soils. They
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