If you're like most gardeners, you want to get the most and the healthiest plants possible out of your space, resources, and time. While potting soil might not seem like a big expense, it adds up year after year, especially if you have a lot of potted plants. Wondering if you can reuse your soil? I've been there! Here's what you need to know about how and when to reuse potting soil.
Can You Reuse Potting Soil?
Yes, you can usually reuse potting soil, but it's not always as simple as it sounds. There are two main problems to think about when reusing your old soil. These are: disease and nutrient deficiency. In many cases, these problems can be overcome, allowing you to use last year's soil to grow healthy plants in the coming year.
I love finding ways to maximize my efficiency in the garden. Dumping potting soil from last year can definitely feel defeating! Whether I buy a potting mix from the store or make my favorite DIY potting soil mix, I don't like to waste.
The key is understanding when you can reuse your own potting soil, when you need to buy new soil, and what you can do to take care of potentially diseased soil or add to the nutritional value.
When Should You Reuse Potting Soil?
In many cases, you can reuse your potting soil in some capacity. Often, you may need to add some organic material or fresh potting mix. Other times, you may need to sterilize the recycled potting soil to make sure it's free of plant diseases that could potentially get passed on to new plants. (We'll go over how to do both of these things below!)
The good news is, there is almost always a use for your old soil. In some cases, it feels like altering the old soil may be more work than it's worth. But once you know what you need to do, you can decide for yourself based on your resources, situation, and gardening budget.
Here are a few situations in which you can easily reuse your soil:
- Plants in permanent pots. For plants that have a permanent container home, you don't need to worry about replacing your potting soil each year. If you notice your container plants become root bound, you can re-pot them. Otherwise, the best way to keep them happy is just to fertilize the soil they already have.
- Large planters. Replacing the soil in large planters can quickly get pricey. Fortunately, for plants in very large pots and planters, it's generally not necessary to re-pot them completely. Instead, just amend the soil by mixing in some organic content to improve the soil structure.
- Potting up new large plants. Potting up larger plants is a good time to use some of your old soil. Just make sure to mix it with some organic matter and/or fresh potting soil, too.
If your situation doesn't fit into one of these categories, you can likely still use the soil. You just may need to do a little more work. Keep reading to find out exactly what you can do to get your soil ready to use again.
When You Should NOT Reuse Potting Soil
There are a few situations in which reusing your soil is just not worth it. If you notice one of these issues, it's best to simply compost the soil. Avoid reusing your soil if...
- It drains very poorly
- It's littered with slugs, insect eggs, or other pests
- You see mold or algae
- It's extremely compacted
How to Sterilize Used Potting Mixes
One of the main risks of reusing potting soil is that it could have diseases in it from past plants. Diseased soil can easily pass harmful pathogens to your new plants. Not worth it!
Sterilizing your soil is especially important if:
- You noticed any signs of disease in the previous plants that used the soil
- There may be weed seeds in the soil
- You're starting seeds or growing seedlings
- You're growing disease-prone plants (such as tomatoes or roses)
There are a few different methods for sterilizing your soil, and all of them take a bit of work and/or time. But, you may be able to yield lots of free soil from the process!
The process of sterilizing your soil involves heating it up to kill any bacteria and potential pathogens, without allowing it to get too hot, as this can destroy the soil structure.
Here are a few methods for sterilizing your soil:
- Black garbage bags. Place your used soil inside of large, black plastic bags and tie them tightly. Then, set them in the sun for 6-8 weeks. The best time to do this is during the hottest summer months. The black bags will heat up the soil and destroy disease.
- Oven method. A quicker way to get sterilized soil is to use the oven. However, it's more difficult to do large amounts using this method. Place your used potting soil on oven trays, spreading it 3-4" thick. Cover each pan with aluminum foil, and bake at 180°F for 30 minutes. Do not let the temperature get above 200°F, as this can destroy the soil structure.
- Microwave method. Another option is to use your microwave to sterilize the soil by heating it for 15 minutes on high power. (This method can be smelly!)
Some gardeners have also used boiling water to sterilize soil from a diseased plant. However, this method may not be as effective, since the boiling water will only hold its temperature for a few minutes, and it may not thoroughly cleanse every portion of the soil.
How to Add Nutrients to Your Used Potting Soil
The other common problem with reusing old potting soil is that it lacks nutrients. Fortunately, it's not too difficult to add nutrients in order to refresh your old soil.
Adding to your soil not only gives your plants what they need to grow, but it can also help rebuild the soil's structure in order to help it effectively hold moisture.
Here are some ideas for organic matter that you can add as soil amendments:
- Worm castings (Get a bag of Worm Castings from True Leaf Market!)
- Aged manure (I personally use Black Kow Brand)
- Homemade compost (Learn How to Start Composting here!)
- Mushroom compost (Espoma has a great Mushroom Compost here!)
You can refresh your old soil in just a few simple steps. Here's how:
- The first step is to place your soil in a large bucket or wheelbarrow.
- Next, remove any plant debris, old roots, rocks, or compacted garden soil.
- Add 1 part compost or organic matter to every 3 parts of old potting soil.
- After that, add a few scoops of slow-release fertilizer, such as Espoma Garden Tone. (Or check out more of my favorite fertilizers!)
- Lastly, mix it all up using a shovel.
What Else Can You Do With Used Potting Soil?
If you don't want to go through the work of sterilizing or re-mixing your soil, don't just throw it out! Instead, add it as a top dressing in your garden beds, use it to fill holes, turn it into your flower beds, or add it to your compost pile.
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