Can you reuse potting soil?
For me, the answer to this question is an unequivocal YES! I've been doing it in my containers for many years, and frankly, I can't even fathom wasting so much good material. Here are three reasons why you can - and should - reuse your potting soil.
1. It's a natural resource. Most potting soils in the US have peat moss as a primary ingredient. Peat moss is an actual species of moss (Sphagnum palustre, or one of many other species of the genus Sphagnum, pronounced sfag-num), and it grows abundantly in wet or moist areas. Under ideal growing conditions, it creates an entire spongy bog, and after many years of growth, a dense underlayer of dead moss forms. This is then excavated and processed into peat for gardens. While peat moss is considered a renewable resource and producers aim to be good stewards of their lands (it is how they make their living, after all), it is ultimately a very slowly renewable resource, replenishing at an estimated rate of about 1mm/.03" per year - that's far slower than the rate at which is currently being used.
2. Disease is unlikely to be an issue. The biggest reason people wonder if it's okay to reuse their potting soil is the fear of spreading disease. However, due to its excellent drainage abilities, most soil-borne diseases are unlikely to develop in potting mix anyway. If you did have a severe issue with a container, you might consider discarding the potting mix in that one, but even then, spread out to dry over winter, it would likely be fine the following season.
3. Disposal issues. In other words, where in the world would you put all that peat moss if you were throwing it out every year? That's a lot of heavy, unnecessary trash! While it could be thoroughly incorporated into a garden bed if necessary, reusing it in your containers the following season not only eliminates the chore of discarding it, it saves you money. Besides, peat decomposes very slowly, so it leaves behind plenty of good stuff for you to reuse.