I have a lot of containers that I'm using for vegetables and am wondering if throwing out used potting soil every year is absolutely necessary? Even if the plant is healthy? What about just adding zeolite and compost?
The health of the soil depends on the nutrients and biological activity - are these all outdoors pots and are you putting new plants in every year, or are you keeping plants alive in each pot year-round?
What kind of potting soil are you using - something you mixed yourself, or a commercial type?
Personally, I tend to do a mix of the old and the new - I empty the dirt from the existing pots (plants dead and gone) into the wheelbarrow then shovel in more dirt, compost, and soil amendments. Mix it around and put it back in the pots. I don't use a commercial potting soil but I try to have enough mulch or compost mixed in so it doesn't compact down too hard in the pot once it is planted again.
If you'll look in the Library you'll find a lot of information on amendments and on making your own potting soil