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Soil types

Distribution of soils | Soil types |Clay soils | Lithosols | Red and yellow earths, ironstone gravels | Lateritic soils | Saline and alluvial | Invertebrates and soil creation |

Distribution of soils

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.

Soil types

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.

Clay 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.

Lithosols

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.

Red and yellow earths, ironstone gravels

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

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.

Saline and alluvial

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 and soil creation

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.