Growing Plants in Containers: Dealing with Spent Soil

Question:

I live in a condo. I once had a big garden, now all I have is a small sunny patio, so I grow everything in pots. I fertilize with liquid fertilizer and grow tomatoes, lettuce, flowers, and even roses.  My question is this: What should I do with the old spent potting soil that I dig out of pots when I replace spent plants like annual flowers or vegetables at the end of the season? I have nowhere to put the old soil and I don’t want to send it to the dump. Can I use it as mulch on other pots?

Answer:

You are correct in thinking that you need to dig out some of the old potting soil and replace with new. About one-third of the old spent potting soil is the right amount to take out and replace with fresh mix before replanting. This is because the old soil is depleted and contains a build-up of salts from fertilizers. Changing the top layer of soil also prevents the build-up of diseases. Don’t ever put this depleted soil on the top of the soil in other containers. This often kills plants, especially geraniums. However, old used potting soil is okay to use as mulch outdoors in gardens and will not harm garden plants.  Sprinkle it on top of garden soil outdoors or use it to fill in a low spot or to replace a watering basin. Since you have no garden I suggest you divide the soil into many small plastic bags, such as newspaper wrappers, and when you go out for a walk take one with you and sprinkle the old soil as mulch over the roots of plants that so often are growing with no mulch over them as a result of “Mow, Blow, and Go” garden maintenance. Often condo gardens have public areas and you can actually improve these by adding your old potting soil to the bare soil surrounding plants. Just keep it away from their crowns (the area where the roots meet the trunk.)