How to Save Old Potting Soil by Recharging It

Container gardening is a wonderful way to grow both food and ornamental plants. Even if you have plenty of garden space, chances are you still have at least a few things growing in containers.

But over time, even high-quality potting mix becomes depleted. The nutrients get used up, the texture breaks down, and plants don’t grow as well as they once did. At that point many gardeners start wondering: can you reuse old potting soil, or do you have to throw it away and start over?

The good news is that you usually can reuse old potting soil. With a few simple steps, recharging potting soil can restore nutrients and improve its structure so it’s ready to support healthy plants again.

But there are few things you need to do to recharge the potting soil and make it fertile again.

Every spring I recharge the soil in our container gardens. Mostly, these are used for ornaments and herbs, but I also have a few fruits, like pineapples, that I grow in containers.

There are a lot of things you can do to recharge the soil, so I’m going to give you lots of options and you can choose the things that make the most sense in your situation.

What I don’t want is for you to go out and buy a bunch of expensive soil amendments. Yes, you may need to buy a few things but when you understand what the soil needs you’ll be able to make better decisions.

Improve Soil Structure

All soil has structure, but not all soil has the correct structure for growing plants. It might be too compact and dense or too loose and sandy…neither of those are good for growing. You need something in between.

Soil structure is the combination or arrangement of primary soil particles into aggregates. Using aggregate size, shape and distinctness as the basis
for classes, types and grades, respectively, soil structure describes the manner in which soil particles are aggregated. Soil structure affects water and air movement through soil, greatly influencing soil’s ability to sustain life and perform other vital soil functions.

USDA Natural Resources Conservation Services

Coco coir

Coco coir is made from coconut husks and is a sustainable alternative to peat moss. Like peat moss, coco coir helps with water retention and aeration. And unlike peat moss which is acidic, coco coir is pH neutral.

Pertlite

Perlite is a made from volcanic glass and is non-renewable and sterile. It improves drainage and aeration and will help prevent the soil from compacting.

Perlite is great for root growth and preventing root rot in container plants. It’s also light weight which makes the container lighter than if filled with all soil.

Like coco coir, perlite is pH neutral.

When working with perlite, its best to wear a mask as it can be quite dusty. Perlite is also void of nutrients.

Vermiculite

Vermiculite is a naturally occurring mineral that expands when heated. It acts like a sponge, retaining water and nutrients and releasing them as the plant needs them.

It aerates the soil, allowing roots to breathe and helps prevent soil compaction.

Like coco coir and perlite, vermiculite is pH neutral and lightweight.

There is some concern about asbestos occurring in vermiculite. So, wear a mask and handle it outside.

Not all vermiculite products contain asbestos, but some do. An EPA study showed some vermiculite products contain low levels of asbestos. Asbestos is found primarily in the unmixed vermiculite product although some was found in pre-mixed potting soils. For consumers using vermiculite on an infrequent basis the health risk is low. Greenhouse workers in daily contact incur a higher level of risk.

Minnesota Department of Health

Add Nutrients to the Soil

The only nutrients available to container plants are the nutrients in the soil that in the container. Once those are depleted, you have to add more. Fortunately, there are several ways to do that.

Compost

Compost is decayed organic material and is full of nutrients and beneficial organisms.

You can make your own compost or buy it already made. You can find bagged compost at most nurseries.

If you buy compost, be sure to buy from a reputable company or farm. We have a local mushroom farm that gives their compost away if you shovel it yourself. If you want their front loader to load it for you, it’s $10 a cubic yard…quite a good deal!

Unlike fertilizer, it will take time for compost to release the nutrients to the plants, which help them have nutrients available longer.

Worm Castings

Worm castings or vermicompost is the waste (poop) from earthworms feeding on organic matter. This waste material is full of nutrients and beneficial microbes.

Since the material has been “pre-digested” the nutrients are available to for the plants immediately but is considered a slow release fertilizer since it will take time for the nutrients to be depleted.

You can set up your own vermicomposting buckets which is great for composting kitchen scraps, or you can buy bagged worm castings.

Because worm castings are so nutrient rich, you don’t need much for each container…1-3 tablespoons, depending on the size of the pot, is plenty.

Bokashi

Bokashi composting is a way to compost kitchen waste indoors. It uses fermentation and creates a liquid fertilizer.

The liquid is quite concentrated and can be diluted with water with a ratio of 1:100 (1 part bokashi liquid to 100 parts water).

The nutrients are immediately available to the plants but don’t last long. It’s recommended that plants be watered with diluted bokashi liquid every 2 weeks.

I use this towards the end of the season when the plants need more nutrients but I don’t want to completely recharge the potting soil.

Slow Release Fertilizer

I’m going to be honest here, I’ve never used commercial fertilizer. I just use combinations of compost, worm castings and bokashi to feed my plants.

However, if I did need to buy commercial fertilizer, I would buy Trifecta+. It’s a well-rounded fertilizer that contains both quick-release and slow-release nutrients. It also has over 70 trace mineral micronutrients and beneficial bacteria and fungi.

Should old potting soil be sterilized?

Maybe, put probably not. If the plants that were in the container previously were diseased, especially with soil-borne diseases, or had pests that overwinter in the soil, you might want to sterilize the soil before recharging it.

However, I’m of the opinion that most old potting soil doesn’t need to be sterilized. When you sterilize soil, you kill any beneficial microbes that happen to still be in there. It’s the nuclear option and is rarely needed.

Sterilize Soil with Water

One way to sterilize soil is with boiling water. Put the soil in a 5-gallon bucket and pour boiling water over it. Stir the soil to make sure it all get wet. You don’t need a bunch of extra water in the bucket.

Cover the bucket and let it sit until the water cools.

The soil, now mud, can be used. However, the soil structure has been destroyed and the nutrients depleted. So you’ll need to add some of the amendments listed above.

Baking the Soil

If you only a small amount of soil you want to sterilize, you can bake it in your oven. Put the soil in an oven-safe container and moisten it. Cover the container with soil.

Bake at 180℉. You’ll need to use a thermometer to check the soil temperature. Once it reaches 180℉, start timing and bake for 30 minutes. Be sure not to let the soil temperature get over 200℉

Solarizing the Soil

A slower but still effective way to kill pathogens in the soil is to solarize it. You can put the soil in a plastic bag, seal it and then set it in the sun for 4-6 months.

How to recharge potting soil

Now that we know the basics of what potting soil needs, here’s how to recharge it.

1. Gather your soil amendments and containers.
2. If the container has a plant in it, you’ll want to gently remove it so it can be replanted.
3. Empty the container into a short bucket on a work table, cardboard or tarp or into a wheel barrow. You need space for mixing. And yes, you need to empty the container because the bottom soil will be compacted.
4. Break up any clumps and remove any large roots. Spread out the soil.
5. Add in the soil amendments you’ve chosen….coco coir, perlite, vermiculite, compost, worm castings, and/or fertilizer. You only need to replace about 10% of the old potting soil with the amendments.
6. Water the soil until it’s fully moist. Use your hands or a trowel to mix it around to ensure it is all damp.
7. Add the recharged soil back to the container.
8. If you have a plant to replant in the pot, add it now.
9. Add more recharged soil if needed to top off the pot.
10. Excess water will drain out the bottom. I like to leave the pot in the bucket for a few minutes to catch the water and any nutrients.

Final Thoughts on Potting Soil

If you’re not careful, gardening can become quite expensive, but it doesn’t have to be. Reusing old potting soil by recharging it is so much cheaper than buying potting soil every season.

You can practice frugal gardening and still have beautiful productive plants!

Thanks for sharing with your friends!