Geoscience Reference
In-Depth Information
Periods of high rainfall can alter the physical nature of soils by causing
compaction layers in some soils and various forms of erosion such as sheet, rill,
gully and tunnel erosion.
On the other hand, long periods of hot, dry conditions can make soil prone to
wind erosion, which occurs when the soil surface becomes denuded of plant cover
so loose soil particles are simply blown away.
Tropical climates are especially harsh on soils, as periods of often intense
rainfall events can cause severe erosion and soil nutrient leaching. Soils may also
be subjected to extended periods of inundation. A large difference between
tropical and cool temperate soils is that in the tropics when it is wet the
temperature is normally high, which encourages periods of rapid plant growth.
This in turn helps protect the soil from erosion and the plants are able to access
nutrients before they are leached out of reach. Soil microbiology is extremely
active during the tropical wet season and dead plant material on the soil surface
will rot down to humus in an amazingly short time, thus the upper organic layers
of the soil are constantly replenished. Tropical jungle soils are in a sustainable
balanced situation and will thrive indefinitely without interference.
Where jungle growth has been cleared for agricultural use the soil is almost
immediately subject to degradation and must be carefully managed to prevent
severe damage occurring from erosion, nutrient leaching and structural decline.
Types of soils
Soils vary due to the parent materials they are made from and where they are
located in the landscape. Broad descriptive terms include: hill soils and river f lats,
light or heavy soils, acid or alkaline soils, for example. Some more specific
descriptions used in the past, such as sandy, sandy loam, loam, silty loam, clay
loam and clay, have been based on particle size. These terms refer to the topsoil
only with subsoils usually referred to as light, medium or heavy clays, further
divided into more descriptive terms such as light sandy clays, medium silty clays
and so on. This system is based on the relative proportions of the soils'
components, with sand, silt and clay as shown in Figure 1.
More recently, soil terminology has become much more specific with the
introduction of the 'Australian Soil Classification System' as shown in Figure 2.
This system considers the topsoil, subsoil, soil chemistry and location of the soil
for the classification. It is a scientifically precise and descriptive system and once
learned allows accurate comparison of soils throughout Australia, unlike the
earlier system. One advantage of this is that if it is found that a particular soil, say
a 'Rudosol', needed specific management in one area it is likely that the same
problem could exist in other parts of Australia as well. This could lead to better
understanding of soils and their management generally.
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