You can reuse the soil. if you use something like a po mix #4 its really easy, Flush, drain, let dry re-use. When i say flush, i mean run water through you containers with soil in it for at least 1-2 minutes, and do this twice. I wouldnt reuse it more than two times though. But, it is a cost saving way
otherwise you would have this GIANt dirt pile outside, wouldnt anyone notice this? plus if we are all thinking the same thing, or should be (how can i do my part to help keep this a green thing) then you should try and recycle things
just my thoughts not anything other than my opinion. The best way to find out, is to try setup one with flushed reused soil and grow another in fresh soil. give the same nutes, amount of light adstrain (clones preferebly) and then document what happens. I found that the plants in the reused soil grew faster, the bud density was identical and the end weight was near the same. So, try it