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Soils of the tropical savannas

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.

cathedral termite mound

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

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.