[Eisadia]

at almost a year too late, i feel like replying (sorry - dont read if you already think you know)

these sorts of questions generally refer to the age and relative stability, tectonically, of the Australian continent.

Fertility is identified under various factors; mineral, nutrient etc.

The prescence of these minerals, which are essential in plant growth, is generally due to tectonic activity such as volcanic eruptions or the interaction of plate boundaries.

I'm not sure about the 'thin layer of soil'.
Dry climate and plant growth are not really related to the fertility of soil, as the nutrients etc. need to be gained from somewhere to be in the plants to go into the soil.


simply.
Plants need nutrients + minerals
No eruptions = tectonic stability = no new minerals
rain and time = leaching/washing away of minerals which make soil fertile