I think the rule is not to reuse the same soil to grow tomato plants (or other Solanaceae (nightshade) family plants like potatoes, eggplant, etc.) in subsequent years to avoid the buildup of fungal spores. However, I wouldn't resuse "spent" soil elsewhere in the garden anyway, especially because potting soil isn't desirable for the garden and vice-a-versa.

Although I agree with the above statement concerning fungal spores, I disagree with the comment about the soil not being desirable for the rest of the garden. The amount of soil in a few pots spread over a garden is so inconsequential in relation to the volume of soil already in a garden in really won't have any affect. There just isn't enough soil in those pots to make a difference in the rest of the garden.

I do this all the time and have so for years because I don't like the alternative of always throwing stuff in the trash. Another alternative would be to toss it in the compost pile which I also do.

Here's another thing I do, but please don't tell anyone. I often use garden soil mixed with some additional peat and sometimes compost for pots used outside. It works just fine. At the end of the season, those pots of soil get dumped right back in the garden. Again, please don't tell anyone because I will probably be scolded to high heaven as a plant abuser.

Kevin

Then, now that's even more confusing to me. I didn't know I had to have special soil for potted plants. Why is that, and why is garden soil different.
This gardening bit might just be to technical for my small brain.
Thanks!

soil is mother earth ...

what is and goes in pots.. is potting MEDIA ...

words mean things ...

in a small container .. the media gets used up ... it simply becomes useless for its ENGINEERED use ... [and dont argue your pots are large .. all pots are technically small.. compared to mother earth]

most potting media is high peat ... its use is to hold water for the plant ... as peat ages .. it simply collapses and loses its ability to hold the water ... and turns mucky .. think walking into a lake and that much that swallows your feet.. lol ..

media also contains other things .... like vermiculite [those shiny things] and perlite [ those volcanic white balls] ....

perlite is there.. to open the soil.. so roots can grow freely ... air is as important as water to many plants ...

the vermiculite does both.. holding some water.. and opening the soil ...

very old media.. can become a gelatinous pile of goo that will not drain properly ... will not allow air in ... and may or may not hold the proper amount of water ...

so.. it should be changed every year or two.. and apparently more often if dealing with a CROP ...

if i were you.. i would leave one .. change the others.. replant the tomatoes.. and observe for yourself.. how the plants react to both .. and then make the ultimate decision after next season ...

i would not throw it away ... i would dump the pots in the veggie garden .. or compost pile.. break them up.. and simply turn the stuff into the soil ... it simply wont be a big enough volume of material to significantly impact a garden bed ... but the downside will be.. that the perlite will be there for a couple million years .. lol ...

finally.. all the power to the poster above.. and if it works.. it works.. but soil.. mother earth in pots.. can lead to severe watering problems.. as the soil can turn to a mud ball.. eventually settling.. and water runs across the top.. and down the sides.. and that can be very problematic for many plants ... it ends up that the center of the pot is bone dry ... but again.. you have multiple pots.. experiment.. find out if you can pull it off.. but the key in that reply.. is that the compost breaks the integrity of the soil.. to allow ENGINEERED water movement.. do not ignore that part of what they said ...

what i have to do.. since i ran out.. is visit the local greenhouses [not many left].. AND TALK TO THE MANAGER.. AND ASK WHAT THEY USE.. AND ASK WHETHER THEY ARE WILLING TO SELL ME A GIANT WHOLESALE[ darn it.. when did i hit caps.. lol] ... bag of media ... A DRY ONE!!! ... the stuff will store.. in a garbage can for a century or so ... and the cost of such.. is usually not much more than 2 or 3 small bags of retail stuff ... but the volume is 10 or 20 times the small retail bags ...

over the years.. i used promix 360.. which i think turned into sunshine something or another ... etc ...

good luck.. experiment.. ken

Thanks ken.

Kevin, I agree that it won't hurt anything. But peat moss that constitutes the bulk of many potting soils err...media (sorry Ken)...is not compatible with clay because it holds water when wet, which is the last thing you need clay soil to do. It also adds no nutritive value to the soil. But yeah, I've dumped it out in my garden with no qualms because it isn't enough to matter.

Just don't dump it in the same place year after year or you will get a soil borne disease like someone I know did and it won't grow anything,