Lane Cove National Park. Photo: Matthew Stevens
Soils of Sydney – Sandstone soils
In the last issue we introduced the five main geological units that make up the Sydney Basin and Cumberland Plain: sandstones, shales, basalt caps and plugs, the Botany Sand Sheet and the riverine alluviums. In this issue we look at the soils that form on the iconic Hawkesbury Sandstones so characteristic of Sydney, the Hawkesbury River, Port Hacking, the Woronora Plateau and the Greater Blue Mountains World Heritage Area.
Sandstone, with its near horizontal layers, weathers in a rather strange way: it cracks both horizontally and vertically to form block-shaped subunits. Over time these either weather down to the original sand particles, which form into a sloping soil layer with a slightly concave surface, or they literally roll down the hill. This can be quite spectacular when a large house-sized block crashes down a slope into a valley, or the block might slide imperceptibly down the slope over hundreds of years.
Sandstone soil horizons
Sandstone weathers to form soils of generally low fertility with loose sandy topsoils and a gradual increase in clay content with depth. These are called “Yellow Earths”. When they are very old and highly weathered in a flat location, or there is a layer of shale in the clay, the sandy topsoil changes abruptly to a sandy clay subsoil. These are called “Yellow Podzolics”.
Soil fertility can vary from very low on hilltops to low or moderate in tall eucalypt and rainforest gully forests. The very-low-fertility soils support plants in the Proteaceae (Banksia, Grevillea etc.) and other phosphorus-sensitive species typical of heath vegetation.
A well-developed soil profile on sandstone has three horizons (horizontal layers):
- A horizon: The topsoil. This has the highest fertility, organic matter content and biological activity. It is always sandy, and ranges from 10 to 40 cm below the surface.
- B horizon: The subsoil. This is typically a yellow clayey sand with little to no organic matter. Its fertility is low. In the Yellow Earths, the clay content is low (<10%). In the Podzolics, the clay + silt content can reach 30%. It ranges from 40 to 100 cm below the surface.
- C horizon: Decomposed rock. This is typically clayey sand to sandy clay in texture and provides little more than moisture storage for plants. It ranges from 1 to 4 m below the surface. Jointing and cracking of the rock beneath (the R horizon) allow root penetration, so fractured rock may allow the development of surprisingly large trees.
The catena
The shape that the soil surface describes is called a catena, from the Latin for chain, because the curve of the surface (in the absence of rocks) is mathematically identical to that of a chain strung between two posts. The characteristic catena down a sandstone hill shows a flat-topped hill with shallow soils, a rocky cliff above a slope, a steep to very steep V-shaped valley strewn with rocks and boulders, some as big as a house, and a creek line filled with a jumble of broken rocks and boulders.
In older and wider valleys the creek may not be V-shaped but will be broad and meandering with alluvial flats, as seen in the Lane Cove River (notably along Riverside Drive).
Hilltop soils
Typically these are the oldest soils in the landscape and hence show the greatest degree of development. A shallow Yellow Podzolic with a greyish brown, sandy A horizon and a very clayey yellow to red B horizon supports low eucalypt woodland. The deeper soil has pieces of bright red to purple, very hard ironstone.
Terraces and rocky outcrops
On steep to very steep slopes, pockets of sandy topsoil to 20 cm deep gradually changing to clayey sand subsoil 20–30 cm deep, often between boulders, support stunted woodland. The shallowest soils support stunted heath.
Upper slopes
On slopes of 15°–25°, Yellow Earths with an A horizon of 20 cm, a B horizon of 30 cm and a C horizon of 20 cm support low open woodland.
Lower slopes
On more gentle slopes of 10°–20°, deeper Yellow Earths with an A horizon of 30 cm, a B horizon of 40 cm and a C horizon of 30 cm support tall open woodland.
Alluvial flats
The deepest soils occur between the littoral (seashore) zone and the lower slopes, on nearly flat land with slopes of 0°–5°. Deep alluvium with an A horizon of 30–35 cm, a B horizon of 60 cm and a C horizon of 120 cm supports tall closed woodland with localised Melaleuca swamps.