Q: Can I reuse old potting soil?
When I started out as a container gardener I could not find literature that addressed this issue, the implication being that one would start out fresh each spring. This is fine when we’re talking about a pair of containers flanking the front door, but when containers ARE your garden, and the soil has to be carried up three flights of stairs without an elevator… I threw caution to the wind that second year and have never looked back. Allow me to share what I learned along the way.
Potted soil is not technically soil at all. It is a close approximation of the real deal comprised of lightweight fillers to aid drainage, and organic matter that provides nutrition and water retention. This substitution is necessary as soil from the garden will become compacted in pots and rot your plants’ roots. Nutrients and organic matter leach out of a pot much faster than they do in the ground or a raised bed. By year’s end what’s left in the pot is primarily filler with very little nutrition if any at all.
In the springtime, just before planting:
- Replenish depleted potted soil with organic matter such as compost or manure. Add no more than 25% organic matter to 75% old soil. Compost is dense, so using too much can lead to that compaction you are trying to avoid. Vermicompost, aka worm poop, is a lightweight, nutritionally balanced additive (approx. equal (N) nitrogen, (P) phosphorous, and (K) potassium) that I prefer for containers. Start a worm bin and make your own!
- Add more fillers such as coir (coconut husk fiber), grit, or perlite if the mix feels too heavy.
- Slow-release fertilizers are also good additives that won’t weigh down the mix. I always add kelp meal by the handful to help plants deal with the stresses of container life as well as crushed eggshells, which add both calcium and act as grit, and fishmeal for nitrogen and healthy leaf growth.
- Diseases can carry over so do use fresh soil with disease-prone crops like tomatoes, or in pots where disease was previously a problem. To avoid spreading disease I practice crop rotation between pots, using the old mix for tough herbs that are better suited to poor soil.
Garden authority Gayla Trail is the creator of YouGrowGirl.com.