i reuse soil, do it all the time, but you have to re-enrich it. put it more bonemeal, perlite, vermiculite, compost material, etc and let it cook for a while (let the microbes multiply). i also add in earthworms (live ones). I use my 'used soil' towards the top half of my pots, and my richer soil towards the bottom half. think of the farmers all around the world, they dont buy new soil, they rotate crops around, and re-enrich their many many acres. over and over.