If I was going to recycle soil, I'd try to have one extra quantity of soil handy and alternate/rotate the use. With the old soil I'd add a bit of compost/guano/worm castings/whatever to it, mix it up and let it sit out a cycle. Then when the next grow is done I'd use it and do the same with the freshly depleted soil. I guess it depends on what you do with your soil when you are growing, but I'd feel more comfortable letting the microbial balance restore itself between grows. My concern would be an unhealthy concentration of something could build, metals, or uhhhh, oh what the hell do I know? Of course if you have a large grow, this makes little or no sense. Although I guess you could keep it outside in a composting barrel.
I wouldn't do it with a cheap soil like MG, it would be hard as hell to guess how to nute when you still have some time release left over. Does MG time release different nutes at different times? I don't know. I don't really want to know, it's cheap to replace.
Another thought, with probably $600-$1000 invested in lights, tent, buckets, ventilation, carbon filter, seeds and nutes, well I don't think I'd try to save money on probably the cheapest component if there's even the slightest chance it could fuck everything up
Just my 2 cents.
