Thornleigh in 1943. Fields and orchards covered the fertile shale-derived soils. SESL now stands near the top of this photo (north is to the right).
Soils of the shale landscapes of Sydney
This is our third article in an occasional series on the soils of the Sydney Basin. In this article we look at the clay soils that develop on shales.
Shale forms the great majority of the geology of western Sydney, but it comes in as close as Ashfield, east of which sandstone predominates. SESL has seen shale soils as far east as Hyde Park, but for the most part, west of a line through Pitt Town, Kellyville, Parramatta, Ashfield, Canterbury, Heathcote and Campbelltown, the soils are formed on Wianamatta Shales. West of the Nepean River the sandstone again dominates the rising ground. Shale also occurs on the ridge capping the North Shore and out to Castle Hill.
In the early days of European settlement the poor sandstone soils of Farm Cove could not successfully grow crops. James Ruse grew the first good wheat crops on the shale soils at Rose Hill, just east of Parramatta. Orcharding sprang up in the Hills District to the north, and cropping, dairying and grazing to the west. The underlying shale made all this possible.
These experiences taught the colonists that the clay soils formed on shales were of much greater natural fertility, and western Sydney became the first “food bowl” for the colony. As a result, much of western Sydney was cleared, and the Cumberland Plain Woodland and Blue Gum High Forest they supported are now endangered ecological communities.
Podzolic soil
The shale landscape forms a typically soft, rolling hilly landscape. The shale weathers to form a soil known as a Podzolic (originally a Russian word) characterised by a fine sandy loam A horizon (the top layer), changing rapidly at around 100–300 mm into a heavy clay. That clay may be red, yellow or brownish grey depending on landscape position: red indicates better drainage (usually at the top of hills), and grey indicates poor drainage (down the bottom of hills). Figure 1 illustrates the sequence.
Figure 1. A typical shale association.
Figure 2 shows an example of an actual Podzolic soil profile from Thornleigh (home of SESL), in northern Sydney, supporting a remnant of the endangered Blue Gum High Forest.
Figure 2. Typified soil profile: Cumberland clay loam on shale.
The Podzolic soils on shale have moderate fertility and reasonably good water-holding ability, but they suffer from periodic waterlogging due to the heavy clay B horizon. Many growers raise the soil into beds to reduce this tendency. You can tell whether a soil has this tendency to waterlogging by the presence of black or red iron/manganese “shotgun” pellets.
Where these pellets can be seen on the surface, this is an indication of sheet erosion that occurred during the great droughts of the late 1800s and early 1900s and the droughts and rabbit plagues of the 1940s and ’50s. Often when surveying soils in western Sydney we see truncated topsoils at the tops of hills and soil profiles further downhill buried by eroded soil from upslope.
The Sydney Podzolics are naturally deficient in phosphorus, but if provided with P can support good pastures and crops.
Being formed on marine shales the subsoils are always sodic and magnesic (they contain high amounts of exchangeable sodium and magnesium) and hence disperse in water. You will often see muddy pools of water on building sites that will not settle out. This is why the Hawkesbury–Nepean River runs muddy after rain and why South Creek is permanently muddy.
For urban development the biggest problem is reactive subsoil (clay that shrinks and swells with drying and wetting cycles).
This important soil landscape is now being built out by massive urban development with little thought to the protection of agriculture and important productive soil landscapes for the future.