I realize that using new potting soil or mixes is the ideal practice but I do wonder if there are folks out there who reuse potting soil. All of my first pottings are done with Schultz Plus or a medium I make myself (from packaged potting soil, Perlite, and spag peat), and all of my plants are from seeds I sow myself. So if the plants are from seeds and the mix is all "clean" to begin with, what are the risks in reusing the same soil in much larger containers (mixing in new as well) as I pot upward for pot-hog plants like cucumbers or Dahlias?
I do realize that the major concern is spreading disease, but I also wonder if this is a small risk or a large one. As i said, the origins of the plants and soil are the same in either case. Also, I'm supposing that replacing depleted nutrients with fertilizers is no different than it would be when a newer soil becomes depleted.
I've already done this with some seedlings so I am asking after the fact, but I am very curious if other people start from scratch as a precaution or if there are others reasons that are not so obvious to me.
I'm cheap, so I recycle it unless it was from a houseplant where I know there was a problem (gnats, mildew). I intend to reuse everything my seeds were started in when I get around to potting them up - hopefully this weekend. :)
I would never reuse seedling mix, I will reuse pots and things (after cleaning thoroughly with bleach), but I will always use fresh mix because I don't want to take any chances. If you do attempt this, make sure you sterilize the used soil before you plant seeds in it. For full-grown plants there's less risk unless the plant that was in the soil originally had some disease problems.
Good point ecrane, I suppose I should amend that I do sterilize the soil before reusing (nuking it in a ziploc baggie in the microwave with a little water added for a few minutes does the trick - hey I think I may have learned that trick from you, ecrane, over on the seed starting forum!). Any dirt that has been with a plant with disease problems goes out the door!