Soils

The scientific study of soil began in South Australia in 1924 with the appointment of J.A. Prescott to the Chair of Agricultural Chemistry at the recently established Waite Agricultural Research Institute of the University of Adelaide. Five years later Prescott became the first Chief of the newly formed Division of Soils of the CSIR (Council of Scientific and Industrial Research), now the CSIRO (Commonwealth Scientific and Industrial Research Organisation). The first modern soil survey in Australia was conducted by Taylor and England. Begun in 1927 near Renmark, and published in 1929, this report still has important practical applications today.

The soil map of South Australia is a simplified version of the Atlas of Australian Soils (Division of Soils of the CSIRO, Ninth International Congress of Soil Science, Adelaide 1968). The soil classification used in its preparation was based entirely on the properties of the soil profile as observed or measured in the field, rather than on inferences about how the soils were formed.

On this map the soils of South Australia are divided into five main groups with sixteen subgroups. The first three groups are the sand soils, loam soils and clay soils which have profiles of uniform texture. The soil group known as earths has gradational texture profiles in which texture becomes finer as clay content increases with depth. The duplex soils have contrasted texture profiles in which the texture suddenly becomes finer (more clayey) on passing from the topsoil to the subsoil.

South Australia is pre-eminently a land of sand and calcium carbonate (lime) with relatively small areas of loam and clay.

The map depicts the broad regional patterns of the sixteen soil subgroups. However, for sound land management it is necessary to know the kinds of soils at the scale of the paddock or farm. To date, this knowledge is available in only very limited areas, as for example in the Riverland and the Barossa district, where soil surveys highlighted the potential of Pewsey Vale for premium stable wines.

The most significant properties of soil for land management are those that influence the acceptance of moisture (either from rainfall or irrigation), its storage within the body of the soil, and its subsequent release of plant roots. Seven factors which affect the availability of moisture and thus place constraints on land management in South Australia are: the hard-setting surfaces which seal readily under the impact of raindrops, leading to erosion in heavy storms (this used to be particularly apparent on the hard red duplex soils during periods of excessive bare fallow); the water repellent character of certain sand soils which are not readily wetted by rain due to certain fungi which 'waterproof' the sand grains; the shallow soil depth; the calcareous material which ranges from hardened sheets of calcrete to loose boulders (many tonnes of stone have been gathered from South Australian farms, and even long after the removal of the last mallee roots, 'stump jump' mechanisms have been required on farm implements to deal with stones); the salinity of the soil; the sodicity, with excess of sodium in the clay fraction of the soil; and the seasonal shrinking and swelling of clays when dry and wet. This last property is particularly present in both the cracking clay soils and the underlying clay sediments of the Adelaide Plains, and has resulted in may building problems in Adelaide and its suburbs.

In addition, South Australian soils in their natural state are notoriously lacking in essential plant nutrients. Innovative work at Roseworthy Agricultural College from 1882 demonstrated the beneficial value of phosphorus for cereal production with the result that the application of superphosphate soon became an accepted farming practice throughout the State. Nitrogen deficiency was recognised but it could not be economically remedied until legumes such as subterranean clover and medics were incorporated in rotations. World acclaimed research into trace elements carried out in South Australia during the 1930s and 1940s led to substantial productivity improvements in pastures and cereals on some of the previously neglected soils of the higher rainfall zones. The most recent development is the recognition of boron toxicity in barley crops which, in trace amounts, is essential for plant growth, but is detrimental in large amounts.

Plant nutrient deficiencies are more readily solved than problems relating to the physical properties of the soils. Land management is difficult when salinity, sodicity, high calcareous content or an excessive depth of sand occurs. When all four occur together as in the calcareous earths and their associated shallow sand and loam soils, land management problems are magnified. This is most apparent in the irrigated areas of the River Murray where salinity problems have been evident ever since irrigation began at Renmark in 1888. A comprehensive pedological and geological assessment of the Riverland region is now needed, which will cover the adjoining parts of the Murray basin in Victoria and New South Wales.

Sand Soils

Shallow sands have uniform coarse-textured profiles, commonly of sand loam, and are found on dunes, plains and low hills of aeolianite and dune limestone. Soil depth is 60 cm or less. The soils are permeable, have moderate to low inherent fertility, and are weakly acid to mildly alkaline. Some coastal areas have been sown to improved pastures, while steeper areas remain under native vegetation.

Deep sands occur widely in the interior, covering the dunes and often the swales of The Great Victoria and Simpson deserts. They are predominantly red or red-brown siliceous sands in which biological and chemical processes are minimal. Various materials, including sandstone, silcrete, clay and carbonate pans, may underlie the soils at depths of up to 3 to 4 metres. Except on the borders of the sand deserts these soils are seldom used.

Bleached sands are developed on highly siliceous parent materials of the older coastal dunes, beach ridges and sand plains of the upper and lower South-East. They have uniform coarse-textured profiles with markedly bleached subsurface sand and coloured subsoil sand. The soils are weakly acid, and experience summer drought and, in some cases, winter waterlogging. Heavy fertiliser applications have enabled some areas to be developed from heath and scrub into pine plantations and improved pastures.

Calcareous sands occur on coastal beach ridges, dunes and sand sheets. They are largely quartzose and contain many shell fragments. The soils are highly permeable and are subject to wind erosion where there is little vegetation. On southern Yorke Peninsula, barley and annual pastures of medics and lucerne are grown on these soils with the aid of regular fertiliser applications

Loam Soils

Shallow loams occur in semi-arid regions, generally on calcareous parent materials. The soils have uniform medium-textured profiles and are coherent when wet but powdery when dry. On the Nullarbor Plain these soils are unused, but in the central areas of the State the native shrub and herb vegetation supports sparse grazing.

Friable loams occur on material derived from basalt and volcanic ash in the vicinity of Mount Gambier. There is considerable organic matter near the surface. The deep brown soil has a crumb or granular structure and is permeable and moderately fertile. The friable loams are among the most highly prized agricultural soils of the State, supporting intensive grazing on improved pastures and the cultivation of potatoes and peas.

Clay Soils

The most extensive subgroup of clay soils is the self-mulching cracking clays on the alluvium of the Murray River valley, the channelled floodplains of the streams draining towards Lake Eyre, and some of the inter-dune flats of the South-East. There are also smaller areas, locally highly significant, throughout the Lower and Mid-North and on the Adelaide Plains, often in association with hard red duplex soils. The cracking clays have uniform fine-textured profiles with significant cracks when dry, although these are not always apparent at the surface. Most of these clays are moderately fertile in their natural state. In the higher rainfall areas these clays support cereals and improved pastures. In the interior, the natural pastures on the clay soil floodplains provided fodder for the flocks of the early pastoralists.

Non-cracking clays occur along the lower course of the River Murray where they support irrigated dairy farming. They are also found in the coastal area near Port Pirie where high salinity limits their potential use.

Earths

Red massive earths have gradational texture profiles that are non-calcareous. They were developed, generally from siliceous parent materials, by deep weathering in warm humid climates before the present arid conditions occurred. They are one of the most widespread Australian soils, but their South Australian occurrence is limited to the plains flanking the Musgrave Ranges and the fringes of The Great Victoria Desert.

Calcareous earths have gradational texture profiles that are calcareous throughout. They are widespread in the pastoral districts of South Australia and the drier margins of the agricultural districts. They have indistinct horizons and are typified by the brownish sandy and loamy soils of the 'mallee' lands. Calcium carbonate (lime) may range up to 10% in surface soils and 60% in subsoils. Some lime is hardened to calcrete (sheet limestone) or may be stony. Natural fertility is low to moderate, but the soils are easy to cultivate and their productivity for cereals in the agricultural regions has been greatly enhanced through the application of fertilisers and the use of medic pasture leys. Surface soils are neutral to alkaline, and subsoils are strongly alkaline. Where irrigated along the River Murray, these soils grow citrus, stonefruit and grapes; the deeper sandier soils are preferred for citrus.

Duplex Soils

Crusty red duplex soils occur in arid regions, usually on tablelands and stony plains. They are often associated with red cracking clays in saucer-like depressions. A surface pavement of gravels is partly embedded in the loamy brown surface soil. There is an abrupt boundary to the red clay subsoil. Through overgrazing, large areas have lost the sparse shrubland that once provided grazing for sheep and cattle, although ephemeral herbs may provide excellent feed following heavy rains.

Hard red duplex soils are probably he most important agricultural soils in the State. Their extent approximately defined the limits of the wheat belt in the nineteenth century. The subsurface soil consists of hard-setting loamy soil ranging from grey to brown to reddish brown. The boundary to the under-lying red-brown friable clay of the subsoil is usually abrupt. Previously known as red-brown earths, the hard red duplex soils are suitable for a wide range of cereal and horticultural crops. Gardens in parts of Adelaide owe much of their productivity and splendour to these soils. Surface soils are typically acid, but may be neutral to slightly alkaline. They are moderately permeable and have responded well to fertiliser applications. Gypsum has been found useful in ameliorating the hard-setting surface soils, and erosion controls, such as contour banks, are often desirable in order to slow rainfall runoff on sloping land.

Hard yellow, brown, red soils with bleached subsurface have a distinct texture contrast between their sandy surface soil and clayey subsoil. The upper part of the surface soil is generally brownish grey, becoming strongly bleached with depth. The upper part of the subsoil is strongly mottled in yellow, brown or red. Inherent fertility is low, and the impermeable clay subsoils often result in short-term saturation of the topsoil following heavy rain. These soils are mainly used for cereals and grazing on improved pastures in the upper South-East and Eyre Peninsula.

Ironstone gravelly soils are associated with dissected lateritic plateaux in higher rainfall districts. Sandy to loamy surface soils with abundant ironstone nodules overlie mottled yellowish clays. Surface soils are acid in reaction. The subsoil clays restrict drainage and produce seasonally perched water-tables on the flatter sites in late winter and spring. These are among the world's most impoverished soils and contain, in their natural state, only 5 to 40 parts per million of phosphorus, whereas the hard red duplex soils contain 200 to 300 parts per million. (As a comparison, in lowland Britain 600 parts per million of phosphorus is common.) These are the most recent South Australian soils to have been developed economically. Large applications of phosphatic fertilisers together with trace elements are needed to sustain the growth of pastures and pine forests.