Potting soil can last for years, but its quality can quickly become compromised in one of several ways. Old soil often lacks the nutrients necessary to grow thriving plants. Also, the texture of old soil can also be less than ideal, and soil-borne diseases and pests can pose a problem. So, even if the soil still looks like soil, it might not be the best medium for your plants.
Lack of Nutrients
Potting soil is depleted of its original nutrients more quickly than garden soil. If it has fertilizer already in the mix, you might not need to add more for the first couple of months, depending on how you're using the soil. In general, though, you will need to feed your potted plants regularly. You can add 1 teaspoon of a slow-release fertilizer for every 1 gallon of soil every three months to make the soil nutrient-rich.
Lack of Texture
In time, potting soil's texture breaks down. A medium with peat, for example, decomposes and becomes finer. This finer soil tends to compact, inhibiting water and air flow, as well as stunting root growth. So, every one to two years, or when you repot, remove one-third to one-half of the soil and incorporate a new potting medium in with the old soil. Then, replant your plants in the mixed soil. You can either save the leftover old soil for future plants, or toss it in the garden or compost pile.
Diseases, Pests and Old Soil
Diseases and pests can be an issue with soil. If your plants are infested with fungus gnats or mold becomes a problem in your soil, for example, you can change potting soils as part of the solution. Or, you can sterilize it. Because of possible soil-borne pests and diseases, your should also sterilize old potting soil before using it again to grow plants.
How to Sterilize Soil
A simple way to sterilize potting soil is in the microwave. Fill a polypropylene bag or microwave-safe container with 2 pounds of moist potting soil. With the bag or container open, microwave the soil on high for 2 1/2 minutes. Wait until the soil cools before planting the plants. Sanitize your microwave before you use it for food. Remember, that if the soil is too fine from decomposition, add new potting soil at a rate of 1 or 2 parts old soil to 1 part new soil.