What can I do with old potting soil?

You can use it again though overtime the nutrients will deplete, you can mix in new compost to restore its value...best of luck!

In addition to the previous post, you can also use it to amend your soil, spread in a planter box, or place it in a your compost bin. If it has never been used, it is still good. I have never had a problem with potting soil 'expiring'.

I use it in the bottom of my pots and add new soil before I plant

i use my used potting mix to fill in low spots in my yard.

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I sometimes do the same as Janet (Smithtown) but more often, do the same as Dfm (Cascade).

every spring i just throw mine in the front garden. when i'm ready to plant there i try and work it in the existing soil

Start composting or raising worms. Mix with seed when reseeding yard.