If more thorough rejuvenation seems in order, start by leaching the old potting mixture to remove excess salts that tend to build up in containers.
1. Put the used soil into a bucket with several small holes in the bottom and fill the bucket with water. Let the water drain out and then repeat this process. After the water has drained away a second time, spread the soil on a tarp in the sun to dry.
2.Mix the leached potting soil with an equal amount of rich, weed-free compost that has been sifted through a half-inch screen.
3.Toss in a light sprinkling of lime and gypsum, and about 1 tablespoon of slow-release or granular organic fertilizer per gallon (4.4 liters) of soil mix.
Add enough water to lightly moisten the mixture; then put it aside in a plastic storage bin or cardboard box to cure for 2 weeks. During this time, the microorganisms from the compost will spread throughout the mix.
When it’s finished, your container plants will have some first-rate potting mix to sink their roots into.
Adapted from The Complete Compost Gardening Guide, by Barbara Pleasant and Deborah L. Martin (Storey, 2008).