Sammy, What I add to old container soil varies and it depends on whether it has been in those containers for only 1 year for two or more. It also can vary a lot depending on what I put in my soil mix when I misxed it up to begin with.

After the first year, I normally add some homemade compost, composted cow manure and then whatever else I think it might need...which might include some bone meal or blood meal, lava sand or green sand. Maybe a little dry molasses. Maybe some partially composted chopped/shredded leaves. In I am growing tomatoes or peppers in those containers, I might throw in a handful of lime. In about Jan., I add all that stuff to the 1-year-old soil-less mix in the containers that will be planted in March or April.

When the container soil is two years old, I dump it out into a new raised bed (since I'm always building new beds) and add all those same amendments again. Then, I mix up a new soil-less mix for the containers.

The two main issues with any soil-less mix in a container is that nutrients get used up and that organic matter decomposes and, to some extent, packs down more firmly which interferes with the permeability of the soil-less mix. So, what you want to do when you're adding to it after one year is to put some nutrients back into it and also some organic matter. Of course, the buildup of salts can be an issue in containers too, but it doesn't become an issue for my plants since I only use soil in a container for two years and I tend to water heavily which must leach out some of the salts. If, in the second year, you're dumping your container mix into grade level beds, you may not need to add much to it, depending on what you're growing in those areas. In raised beds, though, I try to always add the organic matter since the point of the raised beds is to improve drainage in the beds raised above our grade-level thick, red clay.

Some people wait until spring and pour out all the soil in their containers onto a tarp or into a child's wading pool or a very large wheelbarrow, add the amendments, mix them in and then refill their containers. I don't do that. I just add my amendments in small quantities to the soil-less mix in the containers and mix it in well using a transplanting spade.

If you've had a persistent issue with certain diseases that reside in soil, you might be better off putting the "used" soil into the compost pile for a year so composting can kill the pathogens in the soil. I don't usually have much of an issue with diseases in container plants, but that might be because most of our summers in Love County are very hot and dry with low humidity and many tomato-related diseases seem to thrive in moister soil with high humidity.

Dawn

I usually dump mine into a raised bed at the end of the second year. I have some very large containers and they get too heavy to move around as two years worth of stuff starts to pack down in them. Mine all spend the winter in the back yard and so do the chickens. They love to dig in those containers so right now the really big ones are only about half full anyway, and I have dumped the others into a raised bed. With the kind of heavy rain we get, salts build up in a raised bed with an open bottom should just wash right out, if it was ever a problem in the first place.