@ Rbt, add some dolomite when amending old media. @ Air, I reuse my media all the time. I have a base mix of coco, peat and perlite, to that I'll add 30% of new base media, after each use.  Amending it has become a science. If you can test the old medium, that will give you a look into what it needs for the next grow. Never hurts to add bacteria and fungi too, I'm mainly organic though. I'll also send a soil sample in to Michigan State soil lab after every 4-5 times of use and amending. This will point out extreme deficiencies. Peace