Whatever you decide to use...trust me here...

Buy some of the small RED lava rocks.

Line the bottom of your container with it.

It promotes oxygenation in the soil.

It is incredibly rich in minerals.

It aids in watering in so many ways:
A) You can bottom water
B) You can extend time between watering
c) Roots will continue to grow where just soil the will not.
D) Waterlogging is not possible.

You ideally can use many types of gravel, but I have found lava to be the best...mainly becuase of its mineral richness.

You simply wont have mineral problems with RED lava rock being used in your medium.