Does Potting Soil Go Bad?
It’s that time of year again–the weather is warm, the days are getting longer, and your seed order has just arrived. But maybe when you turn to look at your half-finished bag of potting soil left over from last year, you wonder:
Does potting soil go bad?
How old is too old?
Can you still use last year’s soil to start this year’s seeds or to pot up your houseplants?
Is used soil safe to re-use again?
These are good questions, and they are worth digging into (pun intended).
Does bagged potting soil expire?
Short answer: Most potting soil does not go bad the way food does, although it does break down over time and an older bag of soil has lost nutrition.
Long answer: Bagged soil is pretty shelf-stable, but the way you store it matters. If you kept that half-used bag from last season (or an unopened one) in your garage, it’s definitely fine to finish off this year. Garages or sheds protect the soil from big temperature swings that contribute to degradation of the organic matter in the potting soil, and they also shelter soil from snow or rain.
Potting soil that has spent a lot of time sitting around wet or damp is a breeding ground for mold, fungus, and bacteria, which can kill off seedlings and become a nuisance in houseplant pots. If you have some that’s growing a mold colony or has mushrooms popping up in it, you’re better off working that soil into your garden beds. If it has a rotten smell, throw it away, since that odor is caused by harmful anaerobic bacteria.
Potting soil that hasn’t gotten wet or exposed to extreme temperatures, but is more than a year old, is still usable. But because most they have a high ratio of peat moss, which starts breaking down after one season, you’ll want to add back some nutrient content and possibly even some dry weight.
How can you refresh old soil?
One simple way to refresh old potting soil is to mix it into new soil. This is a good option if you know that you’ll be going through a lot of soil anyway. Whenever you open a new bag of soil, mix in a portion of your old soil. It’s best not to add more than one part old soil to four parts new soil–for instance, one fourth of an old bag to one whole new bag.
And if you want to add back more dry weight to your potting soil to make up for the decomposed peat moss, or just to make it stretch farther, you can add rice hulls or fully finished compost.
Another way to re-up old potting soil is by adding “simples.” Simples are one-ingredient fertilizers that are usually derived from natural sources, as opposed to the more complex commercial fertilizer blends.
Great simples to add to old soil are blood meal, bone meal, or feather meal, and even bat guano. We prefer the OMRI-certified products from Down To Earth, which are sustainably sourced and come in cardboard boxes instead of plastic bags.
You can even customize the simples you use for the purpose of your potting soil:
- Bone meal is high in phosphorus and calcium to support root development, excellent for seedlings.
- Blood meal has high nitrogen, which is excellent for foliage houseplants and for leafy greens.
- Soybean meal or cottonseed meal are good all-purpose feeders, along with worm castings.
Mix up to two tablespoons per pound into your old potting soil–a little goes a long way!
So much for old soil that’s been sitting around in its bag. What about soil that has actually spent time in a pot? Can that be reused?
Can you reuse potting soil?
Generally speaking, potting soil is only good for one-time use. The main reason for this is that unused soil is sterile, meaning it won’t be a vector for disease, pathogens, or bacteria that would kill off or weaken plants.
This is especially important with seedlings and houseplants, since they are completely limited to the soil available to them, and lack the resilience that outdoor plants acquire to defend themselves from pathogens. Once potting soil has been used once, it is no longer safely sterile.
However, instead of just pitching old soil from your pots, you can repurpose used potting soil for other uses:
- Work into raised garden beds for better drainage.
- Scatter over flower beds for more organic matter.
- Add to your compost pile.
- Mix into your worm bin.
If you are still worried about old potting soil harboring bacteria or fungus, simply spread it out on a tarp or old sheet in the sun for a few days. The ultraviolet rays will eliminate any pathogens and the soil will be safe for repurposing.