Question: What do you do with the dirt in a pot after the plant is removed?
The short answer is: when I pull one plant out, I put another plant or seeds in the same pot.
Sometimes that is all I do. Other times I dump the entire pot's soil out and amend with rabbit manure or some slow release 10-10-10 then return to the pot if I feel it needs it. Other times, I just top off the pot with potting soil. But if the plant had a large root and I just snipped it off at soil level, I dump the soil into a same sized pot to turn it upside down. That way the fine roots at the bottom will be on the top and much easier to plant. If the soil has become infested with root knot nematodes, I either grow a cabbage family crop in it or I use it on the lawn or to amend resistant plants.
Remember to add some calcium if you are going to plant tomatoes. We gardeners forget about blossom end rot when growing in pots. Adding it after we have the problem is too late.
Rotate plants in pots too... root, leaf, fruit, legume. The exact order is flexible just don't plant the same type of plant in the same soil right away. The previous plant could have harbored disease or insects in the soil that might harm your new planting.
To keep the rotation easy on myself, I use stacked markers in the pot. Cut up old plastic blinds, that people leave at the curb, work perfectly as plant markers. Use pencil on them, face the writing North, and they will last for years.
Seven gallon pots are my preference. Fifteen gallon are nice, but I just can't manhandle it anymore. If you live in Lee County FL and need inexpensive pots, contact Bob and remember to mention me so that my VegHeads 4-H club gets a donation. mrbobiam@aol.com 239.368.3106