The distribution and nature of soils in Australia are related to geological, physiographic, climatic, and vegetational conditions. The widespread occurrence of hard granitic and metamorphic rocks rather than soft sedimentaries has not favored the development of deep fertile soils. The large extent of mountains and hill lands in the moist areas has resulted in rapid soil wastage on steep slopes. Desert types of soils are especially widespread, owing to inadequate rainfall in most of the interior of the continent. Characteristic differences between grassland and forest soils appear in Australia as in other continents. Pedalfer soils are found in the moister coastal zones whereas pedocals occupy the bulk of the continent.
A wide band of podsolic soils parallels the north, east, and southeast coast from the Kimberleys to western Victoria. Tasmania and the southwest corner of Western Australia also have badly leached podsol-type soils. This belt of soils corresponds generally with the more rugged parts of the continent, the wettest areas, and the more heavily forested, districts. These podsolic soils are not very fertile because most plant nutriment has been removed by leaching, and soils do not accumulate easily on steep slopes underlain by hard rocks.
Patches of better-quality soils have developed in the highlands where basalt flows and softer sedimentary rocks of certain kinds weather rapidly into productive soils. Fertile soils are also usually associated wth alluvial deposits along the many rivers that enter the sea along these coastlines.
The Hunter River lowland has long been famous for its productive soils. Similar soils occur along the lower courses of the Clarence and Macleay rivers in northeast New South Wales. Some of the best soils in Tasmania are along the floodplains of the Tamar River at Launceston and along the Derwent and Huon rivers near Hobart. The best soils along the north coast of Australia are to be found along the lower floodplains of the Adelaide, Daly, Victoria, and Roper rivers. Marsh soils along the lower Murray River are notable for their productivity. Coastal lowlands underlain by sterile sandstones, as near Sydney, however, are notably unproductive.
The deep reddish soils characteristic of the basalt areas of northern Tasmania, the Darling Downs, the Atherton Plateau, and elsewhere are also notable for their fertility. The basalt plain to the west of Melbourne, however, is so rocky that it is practically useless for cultivation although it produces an excellent cover of grass. High moor soils, consisting of fine black peat overlying weathered rock fragments and clay, are characteristic of the uplands of Tasmania and portions of the Monaro Plateau. The presence of large tracts of lateritic soils in the southwest corner of Australia represents a fossil soil that developed during a geologic time characterized by much greater rainfall than the present.
Superphosphate fertilizer has assisted in making favorable tracts of podsolic soils somewhat more productive. As soon as the lime and phosphate deficiency is overcome, clover and other legumes may be established successfully. Much excellent pasture has been constructed in this manner along the east coast where rainfall is usually ample throughout the year.
The richer soils of Australia occur in a nearly continuous "horseshoe" belt (open to the west coast) inside the coastal podsol zone. This belt corresponds somewhat closely with the tree-dotted grasslands and brushlands that nearly enclose the central desert region. The rainfall of this soil belt diminishes from the coastal margins toward the interior, as does the quantity of vegetation. Because of the relatively light rainfall, soil leaching is not a problem in this district. Rainfall is the principal limiting factor in agricultural productivity. As is characteristic of most Australian soils, there is usually a phosphate deficiency. In some districts small amounts of copper and zinc sulphate must be supplied along with the superphosphate to correct mineral deficiences.
These important intermediate soils (between the coastal podsols and the interior desert soils) include gray-earths in the north, grayearths and black-earths in the east, and chestnut-earths and mallee soils in the south. All these soils receive annual rainfall varying from about 35 inches to about 10 inches. The humus content ranges from an abundant supply in the black-earths to a scant amount in the mallee soils. Vegetation cover varies from savannah woodland and savannah to mallee scrub. The chestnut-earths in the south represent an extension southward of the northerly black-earths. Summer rainfall is characteristic in the black-earth region, and winter rainfall is typical in the chestnut-earth region. Mallee soils are characteristic of the lower Murray basin, Eyre Peninsula, and the western Nullarbor region. They are light in color, sandy, and subject to blowing when cultivated.
The seasonal distribution of rainfall has important effects upon soil types in Australia. In regions of summer rainfall soils resembling the true chernozem are developed; in zones of winter rainfall soils with surface horizons leached of clay and lime, but still retaining the oxides of iron, are characteristic. In Queensland, podsolic soils usually develop along the coast, black-earths develop inland under wooded savannah conditions, brown and gray soils arise under open grassland and shrub steppe, and desert sandhills lie in the extreme west. In southwestern Australia the normal sequence is forested podsols along the coast, red-brown earths with savannah woodland, pinkish-brown mallee soils, red soils with mulga and saltbush, and desert sandhills in the interior. Lateritic soils occur as cappings of iron oxide and alumina in old peneplain surfaces of low relief especially in Western and Northern Australia.