Hi Starchild....

I am imagining a grow container with lava rocks at the bottom and then soil on top....

is this correct?

If so.. and you wish to water from the bottom...

You can fill a bucket up with 1 gallon of water.. this bucket will need to be slightly bigger than your grow container..

Then put your grow container in the bucket.. hopefuully it will be the perfect size and the water will come up 3-6 inches from the top of the bucket (without overflowing)

Leave your plant/pot in the bucket for 30-60 minutes...

When you return the water should all be gone!! Absorbed into your POT!!

: )