sounds like you used some good medium and yes you can definitely re use soil but you want to make sure to stay organic if doing so or its not that simple. Start reading about TLO or just organic growing and you will start to learn the natural ingredients which make up the good soil. You can easily find the items you need to put back into the soil to allow it to be re used. This is not easily done using synthetics and if your using chemical fertz i would just use the soil in your outdoor garden before growing more weed in it. With organics is very easy to mix natural items back into your soil such as Lime which will balance the PH and add a bit of calcium back to the soil, calcium is one of the main things to be depleted from the soil as you flush most of it out and the plant uses a lot as well. Bone and blood meal is one great way to put the nitrogen back, rock phosphate is good for putting natural trace elements and phosphorus back. The soil is going to be missing things it had when you bought it but if you get it right you can make this soil even better by making it truly living organics which mimics nature more than any other growing method. a bag of lime normally is 40 pounds and I think i paid 6 bucks last time and the same with the rock phosphate. bone and blood meal is very cheap as well along with lots of other items. You will want to add some earthworm castings, 30lb bag of wiggle worm castings is normally about 20 bucks or less and is an excellent nitrogen source you will just need to add a bit of perlite as well as its not airy at all. you can also cut the top off your plants and leave enough on the bottom to reveg the plant and then flower it again with a bigger root ball. If your planning on re using the medium start looking into organics and you will never go back.
