What can you add to the soil to use less soil, in a plant container ?

i crush old soda cans, plastic water or milk contains to fill the bottom, then I cut a piece of landscape fabric a little bigger than the diameter of the pot. This will help the soil from draining out. Then fill with soil. You need to use your judgement as far as how much filler you need at the bottom (plastic items). This works for me and if you're filling large pots they don't become as heavy.

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First I would start with a layer of small rocks on the bottom of the pot then place a smaller size pot upside down over the rocks..........packing peanuts also take up space and allow for water drainage..............fill remaining space with soil.

Styrofoam peanuts in a plastic bag to fill the bottom of the pot.

I use blocks of styrofoam that can easily be cut to pot size. They are easier to clear out when you repot that the small styrofoam bits.

I use plastic water bottles. Works great, allows for drainage and doesn't break down.

Packing peanuts or ghost farts can be placed in the very bottom of the pots.

beer cans :)