What can I use under the soil in my planter?

Christina: Rock would work well for drainage anyway!

Plastic water or soda bottles, rocks or gravel, packing peanuts. Or just buy more bagged soil.

If you have an especially big planter to fill, light, bulky materials are your best bet. Examples include plastic drink containers, milk jugs, crushed soda cans, foam packing materials and plastic or foam take-out containers. Wash the items well to make sure no food residue remains and fill the bottom one-fourth to one-third of the container with the filler. You can use a combination of items to get the best fit. Just make sure the items won't rot. You can also use foam craft balls and blocks from the floral section of a craft store, but these items can be pricey, especially if you need a lot of them.

Gravel or some other type of small rocks.

I use packing peanuts in my hanging plants. Allows for good drainage.

Pebbles or glass pebbles/marbles and they'll help with drainage, too.