What to put at the bottom of container 4 plants ?

[fishy business]

due to having an under gravel filter ive bought a plastic container to put a few plants in to see how they go, what should i put in the bottom of this ?
Sand ? Mud ?

cheers

 

[Rusty_S]

I would suggest sand

 

[Logitech]

I would also suggest sand.

 

[K-Holed]

Gravel should work fine.