Landscape soils

Underneath the Gardens is sandstone and mudstone laid down in the Silurian era (over 400 million years ago). Sands and clays from the Tertiary period (1.8-65 million years ago) are more common in the upper parts of the landscape, while Silurian parent material has produced loamy soils further down.

In 1992, a soil survey was conducted across the Gardens. It found three main soil types: Deep Sandy Yellow Duplex, Loamy Yellow Duplex and Loamy Gradational.

The Deep Sandy Yellow Duplex soils in the higher areas of the Gardens can be prone to water repellency (hydrophobic lit. ‘water hating’) which can restrict the capacity of the soil to hold water for plant growth.

The areas of soil disturbance shown on the map are caused by the changing of the course of the Yarra River in the northern area of the current day Ornamental Lake  in the late 1800’s, and landscape modeling by W.R. Guiloyle through the Fern Gully, Nymphaea Lily Lake and towards D-Gate.

It shows an organic sandy topsoil (to 30 cm depth) overlying a bleached, sandy topsoil (30-65 cm depth) and then an abrupt change soil texture change (duplex) from sandy to a clay subsoil (65-125cm+). Some old root channels can be seen in the clay.

Generally, the soils are fertile and slightly acidic with pH values averaging 6.3. The relatively high organic content of the topsoils provides high water-holding capacities ranging from 32 to 38% by volume. Soils are the foundation of existence for so many forms of life and can be taken for granted under our feet. In the RBG Melbourne, a healthy soil supports the invaluable living plant collections and the historic landscape we all enjoy. Up to 5,000,000 organisms per square meter can be found in healthy topsoil including algae, fungi, bacteria, insects, worms and other life forms.

Worms help reduce compaction, aerate and incorporate organic matter into the soil profile.