Soil Mix

Seedlings: The growing medium used is John Innes number 2 potting compost (40%), washed river sand (20%), fine granite chippings (20%) and Perlite (20%)

1part standard poting soil
2parts of pumice. Pumice (volcanic rock)that is porous. Substitutes include pearlite, non-soluble cat litter, aquatic plant soil or Turface, NAPA oil dry #8822

perhaps 60% pummice/grit/litter to 40% potting soil. 
use 1 bucket (or 1 part) of coir in my mix....

for epi:
1part standard poting soil
1parts of pumice
2parts coarse "orchid bark". This is ground up redwood bark pieces that are marketed for use by orchid growers

Info From KADAS GARDEN:

Mineral Mixes

These mixes are intended for good drainage, and fast drying times. This is useful for species that are from dry areas, have large taproots, and are slow growing. Some examples: Ariocarpus, Astrophytum, Aztekium, Echinocactus, Lophophora, Obregonia, Turbinicarpus etc.

30-60% Grit

10-30% Pumice/Perlite

10-20% Coarse Sand

5% Slow Release/Pellet fertilizer (optional)

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For Water tolerant species

For water tolerant species protected from the rain/uncontrolled water. useful for plants such as: Cereus, Echinopsis, Harrisia, Myrtillocactus, Neobuxbaumia, Opuntia, Polaskia, Rebutia, Stenocereus, Trichocereus etc.

30-50% Coco coir/Peat/Top soil

10-20% Pumice/Perlite

20-40% Grit & Sand

In our greenhouse we use a 60% Coir, 30% grit, 5% pumice & 5% organic pellet fertilizer for all columnar/water tolerant species. Keep in mind in a greenhouse, people control the water.

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For Tropical species

This mix is useful for tropical species that tolerate, and even prefer moist/rich soils. Some examples: Acanthocereus, Hylocereus, Pereskia, Pereskiopsis, Rhipsalis, Selenicereus etc.

70% Coco/Peat/Top soil

10% Grit/Sand

10-20% Compost

5-10% Manure/fertilizer