Here's a link that explains the how & why of Al's gritty mix as it is commonly known. Forgive the punctuation, this is a huge subject =)

The recipe itself is quite simple. All ingredients should be screened. Visually you are looking for bb size to 1/2 bb size. Less than that, the particles are too small & will compact, more than that & the particles will leave air spaces that will be too large.

1 part turface
oil dry from Walmart or Napa oil dry part #8822 will do, use a fine screen and a dust mask, discard the teeny sand like particles, you'll be left with a uniform looking kitty litter textured material

1 part fir or pine bark
pine fines, sold as soil conditioner in certain parts of the country if you can't find it, you will need to screen it, I use 2 screens, first, what passes through a 1/2 " screen is then passed through a regular kitchen sieve & what's left I keep & use, you can use Repti-bark from the pet store it's prescreened, I hear the smaller bags has material that can be used straight from the bag, it's more expensive, I purchased the large bag & screened, ended up with very little, I hear that the larger bags have larger pieces & the smaller bags have smaller pieces, orchid bark can be used too, as long as it's fir or pine & the right size, you're good to go.

1 part grit
can be found at a feed store as brand name Gran-I-Grit, it's actually meant for feeding to chickens to help them grind & then digest their food, be sure to get the 'grower' size, 'starter' (the other size it comes in) is tooooo small.

Don't use anything with oyster shells etc. It should be plain old crushed granite. If you don't have a feed store locally, check places that carry crushed stone, sand, cement, etc. I believe it can be called an aggregate so I suspect it can easily be had there. If all else fails, you can use aquarium gravel, hot pink is preferred (ha ha, just kiddin') get any color you like =) I like to add a pretty tan to make the resulting mix visually interesting. An additional un-needed expense but, hey, I'm a girl =)

Slow release fertilizer, I use Osmocote & pelleted (or otherwise) garden lime @ the rate of 1 tablespoon per gallon of dry mix.

Finally, do a search for any of the terms I've used up above & you'll find explanations that will answer the inevitable questions that will arise =) Here's what mine looks like, Good luck!

Antoinette