What do you guys do with your old "soil mix" in containers from year to year?
Can it be used somewhere else?
What about possible diseases that may be present?
Disease carry-over is distinctly possible. I usually turn old soils into the compost pile or directly into gardens or (more often) raised beds.
There are some readily available guides to the classes of food plants that share susceptibility to the same diseases, and the same practices would apply that apply to beds, which is essentially to rotate to a class that doesn't share susceptibility with the immediate past plant. Of course, that is more a problem with particular diseases and particular classes of plants and the region you're in.
We've had our containers and soil for eight years now. We also compost with a large plastic trash can. (We've had worms in it for the last two years.) Each year, we add soil to our containers, and Oscomote, plus other fertilizers that depend on which kinds of plants we're growing in them. We also add green sand for trace minerals. And, when possible, we mulch with old plant material. (After the snow peas are done in the tomato container, we pull them down and mulch the tomatoes already growing in the container with the pea plants to add the nitrogen back into the soil.) We won't mulch with or stick disease plants in the compost container, obviously.
And, once we get that far, we fertilze every other week according to plant needs.
So far? No problems. We've actually grown tomatoes in the same containers every year (and those are 11 year old containers without changing soil), and still have no problems. After all, what is important? The amount of minerals the plants need. As long as we replenish those, the soil is good.
I have refurbished old container mix for the last couple of years by adding up to one third new fir bark, and a couple of tablespoons of Dolomite lime per gallon. Most times it worked, but in some cases, the drainage was impacted. I will probably continue to use it, but only for the more tolerant of plants, and nothing other than one gallon pots, destined for sale within the next few months. All my one gallon pots include a teaspoon of 18-6-12 time release fertilizer. Al
I usually dump my used container mediums right into my raised vegetable beds. It helps add to the volume, and with the addition of compost and aged manure, keeps the level deep enough for growing every year. I have 4 raised beds that I rotate crops in.
If you're using a gritty mix, you can reuse parts of it. I think some folks screen it and then add some new ingredients to refresh it.