I just usually reuse the old dirt (weeds included) and put it in the bottom of the pot then the new soil is middle to top. I also try to alternate the use of old soil for a different type of plant. Example, my tomatoe soil from last year goes to the bottom pot of my annuals, or other plants. The dirt from my green onions including the weeds goes to the bottom pot of my other plants. Then the tomatoe and other veggie gets the fresh organic miracle grow soil from Costco.
My mom has a farm and she does alternate planting of crops each growing season. After the corn, then the sweet potatoes get planted, then could be another crop (peanuts or mung bean) then back to corn. She believes that By doing this, corn leaves nutrients that is good for sweet potatoe, sweet potatoe leaves nutrients that is good for peanuts or Mung bean and these in turn will leave nutrients that are good for corn (and the cycle continues).
So I try to use old soil for a different plant each time.
