Dear Jim:
Subtropical and tropical soils are extremely well drained,
mostly acid and rich in iron and aluminum. I would use a mix of 1 part
coarse perlite (sieved to exclude dust), 1 part commercial compost, 1 part
crushed rock . For big sized species like saundersiae, oppositiflorus,
crassifolius, daleni, garnieri, etc. a 5 gallon container would be enough
for the deep planting necessary for successful cultivation. Of course,
several good sized drainage holes are a must. Heavy fertilizing must be
avoided but monthly waterings with a very dilute iron sulphate solution have
proved very useful.
Regards
Alberto