It is on this colluvial and alluvial material that the Gardens are located. Geomorphology Soils are generally grey clay loams over heavy yellow clays. The loam is bleached and sets hard on drying. The clay is densely packed and has restricted drainage. There is a distinct boundary between the loam and the clay horizon.
Rocks are very common through the profile, reflecting the colluvial nature of the parent material.
The soils are of inherent low fertility with deficiencies in nitrogen, phosphorus, calcium and a range of trace elements. Structure is generally poor to moderate. Erosion can be a problem where the soil is not protected by vegetation. Intensive horticultural use requires some fertilisation and protection of soil structure to overcome these limitations. The mean annual rainfall is 655 mm in the Gardens.