We are preparing our gardens and pots for winter now. It seems a waste to throw out depleted potting soil, but is it safe to reuse it? And if so, how? There are a variety of answers and ideas out there, so let’s get started.
First of all, yes, you can reuse old potting soil. But, and this is a big BUT, you need to approach the potting soil reuse knowledgeably. Not all potting soils are created equally, but even those higher quality ones will most likely be nutrient depleted at the end (if not before) of the growing season. Organic potting soils especially may suffer. “In conventional potting soil, synthetic fertilizer can provide most of the food a plant will need for a while, but organic growers must rely on compost, which often doesn’t provide enough nutrients  to last long in a pot” writes gardening expert Craig Idlebrook.
If you hope to reuse potting soil for your next growing season, you will need to choose a way to replenish nutrients. One way is to compost. Dump your old potting soil into a bin or pile and add manure (chicken manure in moderation is perhaps one of the best), leaves, grass clippings, coffee grounds and old vegetables. Composting doesn’t work as well over the winter, since decomposition slows down when the temperatures chill. Plan to leave the bin or pile composting over the coming summer.
Or, if you have a method of keeping your bin warmer over the winter, it may be ready in the spring for potting. Some good ways to achieve this are to put a roof over it (to keep the rain and snow from chilling it), block the pile in with walls, and lay a tarp over it. This keeps the pile warmer. Also, make a bigger heap. The inner layers of the pile will continue cooking even in colder temperatures. The bigger the pile, the bigger the inner layers available for springtime use. If you aren’t able to compost, still dump your old potting soil into a bin. In the spring, you can mix the old potting soil with new. There’s no magic formula; some people do half and half. Be prepared to give plants additional food as needed over the summer.
Secondly, when reusing old potting soil, you need to be aware that soil can carry plant diseases, non-wanted seeds and bothersome pests. The longer a soil has been in use, the greater the chances that it contains some plant-harming substances in it. Sometimes leaving your soil out in freezing temperatures over the winter is enough to kill off any unwanted pests. You can also bake your soil or “nuke it” in the microwave. These methods will also kill beneficial bacteria, but the pros can outweigh the cons.
If you wish to take a more relaxed approach toward your soil, you can evaluate each pot at the end of the growing season. If the plant displayed disease, then toss that soil or nuke it. If the plant seemed healthy, then potentially that soil is okay. In the spring, when you begin repotting, use the older potting soil for hardier plants, like some herbs. Use fresh, clean soil for disease-prone plants like tomatoes.
But what if you aren’t interested in reusing the soil for growing? Do you just throw it out? Hardly! An excellent use for old potting soil is for potting plants that you swap with others. The soil only needs to hold a plant secure generally for a short time before it reaches its new home and is planted in the ground, so the nutrient deficiency isn’t noticed by the plan. You can also use old potting soil to fill up potato bins. In the fall, toss your old potting soil into a bin and throw in some manure, shredded leaves and compost. In the spring you’ll have a nice barrel mix for potato growing. Old potting soil can work well in lasagna gardening as well.
I am sure other uses for old potting soil have been discovered, so please share in the comments below. In the meantime, let’s be frugal with our soil and cut down on the cost of outfitting pots next spring!