The soils of the tropical savannas, along with the
distinctive wet/dry climate, are a major determinant of vegetation
in the region, and of potential land uses. Soil is an outcome of
five broad factors: parent material, climate, relief/slope, time
and organisms.
Termites play an enormous role in breaking down
plant material. This process is essential if nutrients are to be
returned to the soil environment.
Photo: Peter Jacklyn
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Infertile soils
Given the variations in all of these in the savannas it is no
surprise that there are many different soil types in evidence in
northern Australia. Generally speaking the combination of these
five factors has resulted in soils which are characterised by their
infertility, although there are exceptions. In other words, very
old and already well-weathered parent material has in many areas
been leached of nutrients by the very high summer rainfall. Those
areas of greatest wet season rain tend also to be the most
infertile. Lateritic soils for example, which are the most
weathered and infertile of all soil types across the tropical
savannas, are found in areas closer to the coast with high summer
rainfall.
Nutrients levels in savannas soils are further reduced by
aerobic bacteria which are particularly active in areas where the
temperature remains above 25oC degrees for substantial
periods of time. The activity of these bacteria under such
conditions occurs at a rate above that of plant growth, thus
breaking down plant matter faster than it is produced. The result
is that soils are lacking in humus which contributes much of the
nutrients to soil under different conditions.
Variation in soil factors
Of course, soil characteristics are highly dependant on
localised factors and so can vary considerably within a small area;
there are probably different soils within an average paddock for
example.
There are however a few generalisations to be made about the
soils of northern Australia:
- soils are more fertile in the south-east sector of the tropical
savannas while in the north-west shallow lithosols (skeletal soils
in which only the 'bones' of the soil remain) and infertile deep
sandy soils dominate.
- predominance of poor fertility across the tropical
savannas
- concentration of fertile cracking clay soils in the inland
drier regions.
Soil type
Soil is essentially a scab-like crust which forms on rocks as
they decompose under the elements. The parent material from which
it forms provides the basic mineral elements for the soil, thus
soils formed from granites tend to be sandy and infertile, while
those formed from basalts will be more fertile and clayey. The
particles may then be further reworked through the wind, or by
gravity. Deserts, for example, consist largely of sand because the
other lighter particles have been blown away; valleys often contain
deep silty soils which were washed or tumbled down from surrounding
hillsides. Climate, particularly the amount of precipitation, is of
primary importance. Thus tropical areas of high precipitation have
the most intense chemical weathering conditions on Earth. In this
context though it must be stressed that there exists a great
diversity of soil types in northern Australia.
Chemical process
So the process of soil formation is essentially a chemical one,
although physical elements also play a part. As a rock surface is
weathered, certain chemical elements are leached out. Soil horizons
(layers) are formed because the top layer (the A horizon) loses
elements, and deposits them below in the B horizon. The B horizon
then is often, but not always, markedly different from the A
horizon—this sometimes explains why the mounds surrounding
ant nests are so often different in colour to the soil surface on
which they sit.
Parent material, speed and climate
The kind of soil which forms depends on the parent material and
the speed of the process. The temperature is also significant. In
much of Australia, the rain falls during the cooler months. In the
north however, warm rains falls during summer. Things dissolve more
quickly in warm water, and rocks are no different. The volume of
rain is also important, for it means that the water table
fluctuates through a greater range each year and rises quite close
to the surface. Some areas (such as the floodplains of Kakadu)
become waterlogged for months on end. All of these factors play an
important role in soil formation. Vegetation too is important as
plants release acids which act to break down rocks. Vegetation also
contributes organic matter to the soil which can affect the
nutrient levels and the soil structure.
Soil classes
Soil is a three-dimensional concept, and does not simply refer
to the surface matter. Any soil class incorporates an A, B and C
horizon, all of which have particular characteristics. The class
will also refer to proportions of different sized particles
(grains) and organic matter, which together determine many
important soil characteristics. Sand is the largest particle size,
clay the smallest and silt somewhere in between. When people speak
of loam, they refer to a soil which has a balance of the three.
Sandy soils generally have lower-water holding capacity and higher
water infiltration rates compared to clays. Loams combine the
porosity of sand with the water-holding ability of clay and are
thus considered the optimum soil texture for cultivation.
