“Yes, I know, but I have 15 or so Earthboxes, each with about two cubic feet of promix, and the Earthbox folks say it can be reused for years. I do mix it with fresh, and dump some of the old on my yard as mulch each season ( I do two or three rotations a year) . I can’t fathom dumping it all and starting fresh every year. I would be dumping about 13 bales of Promix! Seems so wasteful, and that doesn’t fit with my idea of gardening.”
Ah, now I have a better picture of what you are doing. I managed a large number of big to huge pots for a large estate and reused the soil in those pots, but there are tricks to this and there are times when the soil has to be composted.
Pro Mix, or any mix will break down, get mucky, lose nutrients and become acidic over time. The latter may not happen if your water is alkaline. Mixing in some aged compost annually can help with the nutrients. Seaweed extract and Plant or Garden Tone (Espoma) can also add broad-range nutrient boosts. Perlite can aerate and prevent/correct mucky soil whose particle size is too small. Dolomitic lime can correct acid soil. For the latter, you’ll need pH paper or a good meter. Don’t waste your money on those cheap meters with two metal probes that go in the ground; I’ve never seen one that worked.
Small plants may have trouble going into used soil that has no resting period. Before replanting, it’s best to let large containers sit empty for at least a week or two and approach drying out; this creates an unfavorable soil environment for pathogens. The pathogens starve (no food) and get stressed, creating an environment favoring microbes that eat pathogens. During this time, the plants to be put into the large containers are staged in smaller containers, usually up to 4 inch (10 cm). It’s best if each growing season has an unrelated plant type=rotation. However, the soil can become infected despite the best practices. In these cases, it must be discarded, ideally deeply buried and/or sanitized.
Signs of pathogen contamination are usually obvious. Sclerotium fungus makes tiny, mustard-seed like growths near the soil-plant stem line. Many blights, like early blight or late blight cause quick yellowing of foliage, followed by stem collapse. If you see these things, keep in mind the stuff can spread, on your hands, by water. Carefully bag the soil and bleach the pots, far from other containers. The soil can be sanitized if one has hot sun and a solar oven, but that’s not a common thing. We built one which melted a compost thermometer! An oven thermometer read over 180 F (82 C) for over four hours.
Your goal, aside from keeping soil chemistry good, is to keep the bad guys out. To this end, it may be helpful to use Actinovate on a regular basis. This is a biological, Streptomyces lydicus, which can help protect plants from diseases. It’s not cheap, but it can help. New biologicals are being developed all the time. Search “organic fungicides” or “biological fungicides”. Always follow product labels. Most of these are listed as organic.
Much of the other successful practices are obvious: don’t leave dying plants in containers, remove as much of the root ball as possible, don’t overwater, insure good air flow, manicure plants to remove dead/dying leaves or stems, etc.
