Native to central Mexico, Ferocactus robustus grows in dry, exposed places where rain moves through the ground quickly and roots are surrounded by air. That background explains its preference for sandy, loamy, rocky soil rather than dense, water-holding potting mixes.
The best soil for Cactus is sharply drained but not sterile. A mineral-heavy texture helps excess moisture leave fast while still giving the roots enough contact with the soil to anchor and take up water after brief wet periods. Slight acidity to neutral conditions suit its natural soil chemistry.