The soils of Melbourne have been formed by complex interactions between climate, topography, organisms and the rocks constituting the upper mantle of the earth's crust. The soils have developed for varying time periods, ranging from millions of years to a few thousand years. Geology and topography provide the initial keys to understanding why particular soils occur where they do in the Melbourne area. The Flora of Melbourne (1993) includes a map of Melbourne soils produced by staff of the former Soil Conservation Service of Victoria. The soil classes are based on soil texture, a property inherited mainly from the underlying rock material.
Many Melbourne soils have three obvious horizons: a pale-coloured A horizon (save for a darker zone of organic matter accumulation at the top), overlying a yellow, brown or even red B horizon, which grades into a C horizon or zone of weathering parent material. Usually, the A horizon has more sand-sized particles than the B horizon, which has more clay. Such soils are called 'duplex soils' to connote the marked contrast in texture between the A and B horizons. The yellow to red colours of the B horizon indicate a concentration of different types of oxides of iron, some of which have been washed into the B horizon from the A horizon, leaving it pale or even bleached in appearance. A common feature of duplex soils in the Melbourne area is that the A horizon becomes waterlogged in winter. Under such conditions, normally insoluble ferric iron is reduced to ferrous iron and dissolves, only to be reoxidised and reprecipitated when the soil dries out. The result is a layer of concretions of iron oxide gravel or 'buckshot' that builds up at the A-B boundary. Examples of these duplex soils are common in the maturely dissected land surface on very old Silurian sandstones and siltstones in the eastern and northeastern suburbs. If the A horizon is removed by erosion, the gravel layer then appears at the soil surface, a common phenomenon on soils from which the vegetation has been removed as part of land development.
The profiles of present-day soils reflect the action on parent materials of climate and other influences since the last glacial period at the end of the Pleistocene, some 11 000 years BP (before present). During the alternating glacial and interglacial periods of the Pleistocene (c. 2 million years BP), the earth's temperature fluctuated several degrees, and the mean sea level rose and fell by many metres. Extensive sand dunes were formed along the coasts in the colder periods, but as the climate warmed and the sea level rose, the dunes migrated inland. Soils formed on older surfaces and subsequently covered by new materials are called 'buried' or 'fossil' soils. An example can be seen at Diamond Bay Beach on the ocean side of the Nepean Peninsula, where soils called terra rossas and rendzinas are buried under unconsolidated recent dunes. A terra rossa has a warm red colour, darkened by organic matter near the surface, and a gradual increase in clay content with depth. This soil usually forms by deep weathering on calcareous parent materials, whereas the rendzina is a shallower soil comprising a dark, organic matter-enriched A horizon overlying fragmented, weathering limestone. Both soil types are favoured for viticulture. The terra rossa should not be confused with the krasnozem, which is a deep, red, well-structured soil that typically has formed on volcanic rocks, such as the rhyodacites of the Dandenong Ranges, under high rainfall. Although acid in reaction and requiring large amounts of phosphate fertiliser, krasnozems are highly prized soils, being used for horticulture and vegetable- and flower-growing in the outer Melbourne regions of the Dandenongs and Yarra Valley to the east, Kinglake to the north and Mornington to the south-east.
Other igneous rocks in the Melbourne region were formed from basaltic lava flows that occurred during the Tertiary and from the late Pliocene to Pleistocene periods. The soils formed on these flat or gently sloping land surfaces are typically black earths that are dark grey to black in colour and of uniform texture down the profile. They are normally neutral to alkaline in reaction, with a clay content as high as 60%, which makes them sticky when wet and subject to cracking when dry. Extensive exposures of the 'Newer Volcanics' (less than 7 million years BP) occur in the west and north-west of Melbourne and in the valleys of the Darebin and Merri creeks. They are more widespread than those of the 'Older Volcanics' (40-20 million years BP) that outcrop at Flemington and Lilydale and on the Mornington Peninsula. The land surface during the Tertiary was also subjected to prolonged periods of erosion, as a result of which thick sedimentary deposits formed in the Port Phillip Sunkland. These range from coarse sandstones (for example, Red Bluff Sands of the Brighton Group) to marine clays and marls such as the Balcombe Clay. Soils formed on the moderately dissected sandstones of the Brighton Group are widespread in southeast Melbourne. They are very well drained (droughty in summer), acidic and low in fertility.
Fertile, loamy soils occur on alluvium deposited in present river valleys, such as those of the Yarra and Maribyrnong rivers, but they suffer from periodic flooding. Alluvial deposits also occur in the Yarra Delta and overlie estuarine deposits such as Coode Island Silt and Fishermans Bend Silt. Coode Island Silt is particularly extensive, and in many places contains sufficient iron pyrites to produce an 'acid sulfate soil' when drained and exposed to oxidation.