| In the wild it depends on the species... A great number do grow in shale and in rocky outcrops. About the next most common soil is sandstone/quarzite soils (which are often also rocky); often in montane regions, other times as low down as coastal sands (eg. P. scolymocephala). But others are found in grassland (eg. P. caffra, P. welwitschii, P. dracomontana, etc.) and grassy montane slopes (eg. P. lorea etc.). Others come from moorland (eg. P. asymmetrica, etc.). Others are happy in a wide variety of montane habitats (eg. P. nana, etc.). The root-run of plants in the wild is of course in theory unrestricted. The roots can penetrate deep between fissures in shales and rocks and seek out purchase, moisture and whatever minerals they can get their hairs on. Runoff water from montane slopes (over shale or into rocky fissures) will also contain soluble minerals (surface materials such as animal remains/faeces and decomposing vegetative produce). The absence of an actual organic layer in the soil itself does not mean the soil is barren nor that the water which reaches the plants is inert. When growing in pots the roots are very very restricted compared to this. We therefore have to compensate for this by adding increased organic matter to the soil to improve its water holding (and importantly, also therefore the soluble mineral holding) capacity. A plant in a pot has very limited root run compared to the wild, and to plant it in pure rock would mean it had virtually no chance of obtaining sufficient soluble minerals (or water indeed, unless we watered a few times every day) for its needs. As an example, I recently repotted some Aulax plants from 3" pots into 5" pots. The plants are around 4-6" tall. The roots, when laid out during repotting were three feet or more in length. These plants come from sandy soils and use these roots to make sure they can get enough of what they need from the soil. | 
