Potting soil is expensive—and heavy—so most container gardeners put some type of lightweight filler in the bottom of planter boxes. This allows the soil to drain properly and also encourages healthy plant root growth. It also helps keep non-biodegradable plastic out of landfills.
Examples of good container garden fillers include:
Plastic water/soda/milk bottles (with caps on, to prevent water accumulation and possible bacteria or mold growth)
Old plastic pots turned upside down
Styrofoam packing blocks and foam packing peanuts (the non-dissolvable kind)
Aluminum cans
Old swimming pool noodles
Old plastic colanders
Pinecones