It depends on the quality of the potting soil. Potting soil does deteriorate and compact over time with use and frequent watering and when that happens, you lose the fast drainage and aeration that are key to successful container growing. It is for that reason - rather than disease issues (which tend to be far less common in container growing situations) - it is suggested that container soil be replaced each season.

If your soil is something like Miracle Gro (very finely textured) that doesn't hold up well over time - MG is most definitely a single season product - then you are risking the health, quality and productivity of your plants next season. If it is something much more durable and textural like the 5-1-1 or gritty mix referred to on the Container Gardening forum, you are in much better shape and can refresh for another growing season.

If your container plants DO get disease issues then by all means, you want to completely replace the soil.

Thanks for reply. I wasn't quite sure why soil used for tomatoes in particular could not be reused two years in a row compared to other vegetables. I understood the loss of nutrients and the compacting of soil, but it seemed that it was just a given no when it came to tomatoes. The potting soil I use isn't Miracle Gro, though it seems to drain well even now. At the end of the season, I plan to amend the soil with mulch (pine mostly), coffee grounds and egg shells over the winter and add vermicompost in the spring. If the soil seems too compacted, I'll add some perlite. Does this sound feasible? Also, no disease issues so far, just a few white flies.

Blight is probably what is making people recommend not to reuse the potting mix. The spores that cause tomato blight (the late blight that crawls up from the bottom and makes the leaves turn brown and fall off) live in the soil and are spread to the leaves when rainwater splashes soil onto the bottom leaves. I have reused potting mix for tomatoes by mixing in some fresh compost and using it in the bottom half of the large pot. Then finish with fresh potting mix on top. My theory is the plant is less likely to get those spores if the reused soil is covered. My tomatoes always seem to get blight no matter what, but it was certainly no worse than usual. Compaction is a real problem so if you try this, use very coarse compost or some shredded wood like you would find in coarse potting mixes.

Any leftover mix from last year ends up in my compost pile to (hopefully) re-equalize the spore population. I also mulch around tomatoes as soon as they are planted to reduce the soil splashing.

Thanks, toxcrusadr! I feel more confident about reusing the soil. If I do end with some diseases, I'll reconsider reusing the soil, but as of now my only problem is a few white flies. Also, thanks for the explanation. I couldn't find a clear explanation why tomato soil in particular could not be reused for tomato plants two years in a row. What you and gardengal48 said about diseases makes sense.

In the garden it's also good idea to rotate locations of tomatoes, especially if you get blight.

This is a good conversation....I have a raised bed that seems to have blight in one spot. The situation tox described above is exactly what happened to the tomatoes in that section of the bed. Now NOTHING will grow in that spot. If I remove all the soil, down to ground level, will this get rid of the blight spores or is there something else I should do/use? [maybe I should start a new thread. Is this hijacking???] Thanks folks, there is so much good wisdom here from y'all.

Thanks tox.