Distribution of soils | Soil types
|Clay
soils | Lithosols | Red and yellow earths, ironstone gravels
| Lateritic soils | Saline and alluvial |
Invertebrates and soil creation |
The exact distribution of soil types in the savannas is
complicated, and related to geology, geomorphology and rainfall
gradients. In some areas plate tectonics have broken up the
original ancient plain, and exposed the bedrock underneath. In
others, large-scale deposition of marine sediments over vast areas
has occurred. In some coastal areas, relatively recent uplift has
exposed alluvium and estuarine deposits.
Major soil types of the savanna zone include:
- lithosols
- lateritic soils
- cracking clays
- red/yellow earths
- deep sands
- alluvial soils
For more detailed information on soil types, click
here to go to our table on savanna soils.
As one moves inland soil textures change, although not on a
uniform gradient—sands, clays and loams appear throughout the
region, regardless of climatic gradients. However, high rainfall
zones along the coasts tend to have less clay/silt in the soil as
these get washed out. Clayey soils, which tend to exist in areas
further inland, can be found in the Victoria River District, the
Barkly Tablelands and the Mitchell Grasslands.
The clay soils of the Mitchell Grassland are one of the most
fertile soil types found in Northern Australia. On the eastern side
of Mitchell Grasslands, there are vast clays pans which are
remarkably uniform over large areas and have an average depth of
about 1 metre. Clay is a curious soil because of its chemical
structure. In essence clay is a very unstable substance and wants
to break apart. On an atomic level the particles which make up clay
repel each other, because they all have negative charges. These
negative charges also enable the clay soil to hold on to positively
charged nutrient elements such as potassium and magnesium. It also
has an enormous capacity to hold water, again because of its atomic
structure, but will relinquish the water in drier times. Some kinds
of clay can expand to between two and 10 times the original volume.
On drying, huge cracks form in the surface which provide a unique
habitat for fauna.
Much of the Kimberley-Arnhem areas consist of rugged escarpment
country with soils dominated by lithosols. These soils can also be
found near Mount Isa, and adjacent to the fertile cracking clays of
the Mitchell grass region. Lithosols are in fact common right
across the tropical savannas in patches, and tend to be shallow and
generally infertile. Around the Gulf country there are alluvial
soils made up from parent material of various ages, saline
mudflats, clay soils and sand dunes closer to coastal areas. On the
west side of Cape York, there are massive red and yellow earths
(soils without horizons which occur when the soil is formed very
quickly) which are fertile but not friable. North East Queensland
has diverse soils as a result of its geological complexity, and the
great variety of topography.
In the Northern Territory, soil types range from massive red and
yellow earths to shallow ironstone gravels. There are considerable
areas of shallow stony and sandy soils interspersed with massive
red and yellow earths throughout the top end. Surface textures
range from sands to clay loams. Most soils are permeable in the
surface layers but increase in clay content at depth. Black and
brown cracking clay soils occur to a limited extent throughout the
top end, but are common on the seasonally flooded coastal
areas.
Lateritic soils are found right across the tropical savannas.
They tend to be moderately acid on the surface, and increasingly
acidic with depth. The nutrient content, and biological activity,
are very low in these soils, and they maintain only a thin A
horizon of low organic content. They are often characterised by the
presence of hydrated oxides of iron and maybe Al, which remain in
the soil after other elements have been removed. In the top end of
the Northern Territory these form ironstone gravels either on the
soil surface or in a layer between the A and B horizon, or it forms
`coffee rock', an extremely hard and un-erodable rock. In Western
Cape York it forms the basis of the bauxite deposits. Beneath the
laterite there tends to be a layer of yellow or red claystone which
are often quite fertile. Laterite is the most eroded soil possible,
and this is why great chunks of the stuff are used around Darwin as
monuments — the rock cannot decay further.
In coastal areas south of Darwin, in Kakadu and on coastal areas
of the Gulf of Carpentaria and west coast of Cape York, soils tend
to be saline. Alluvial soils are found in floodplain areas;
generally such soils are quite rich, as they have been washed down
from higher areas and deposited in a floodplain or delta.
Invertebrates are critical in both soil creation and nutrient
recycling and indeed much of the soil now present in northern
Australia is said to owe its existence to several thousand years of
termite activity.
