The dangers of reusing old potting soil


It’s so tempting to leave the old potting soil in the container and reuse it next year. After all, the potting soil wasn’t cheap, and it’s supposed to be filled with nutrients to promote healthy plant growth, so why not reuse it?

How to reuse old potting soil? Come winter, dry out your disease-free potting soil and store it using containers in a cold spot. In Spring, solarize the soil. Afterward, reintroduce essential nutrients and minerals using fertilizers or compost. Your potting soil is all set to grow.

Re-using old potting soil isn’t always a good idea let’s first look at some cases where you should leave the old dirt alone.

Why Reusing Old Soil Isn’t Always A Good Idea

Old potting soil usually has been depleted of nutrients and has nothing left to feed the plants. If the plant has enough good potting soil left around its roots in the nursery container after being planted it may survive for a short time. Minerals might also be lacking in addition to the nutrients. Without minerals, the plants will become weak, diseased, deformed, and die. Unless you’re willing to invest the time and effort to clean, solarize, and replenish the nutrients in the old potting soil, it’s not a good idea to reuse it.

If the plant(s) in the potting soil did not produce as expected, were deformed, or died a premature death, you should not reuse the potting soil. Viruses, bacteria, and fungi will live on in the potting soil long after the plants have died. These plant pathogens will overwinter in the used potting soil and attack new plants next spring. It may be possible to destroy these lurking pathogens that have remained in the potting soil, but it’s not worth the risk. The solarization process will kill these diseases the majority of the time, but if you know plants were killed last growing season by something in the potting soil, discard the used soil.

Pests are very adept at laying eggs in secure, undetectable locations, and what you may consider being a normal part of the potting soil may be pests eggs. If a pest infestation destroyed your plant, don’t save the potting soil it was growing in and discard the container as well. The likelihood of pest eggs/larva being in the old potting soil and container are high. Even though the solarization process and container cleaning should eliminate all pests and eggs, the risk is not worth reusing the old potting soil.


Can You Reuse Old Potting Soil

The short answer is ‘Yes, but…..”. The old potting soil can’t be left in the container throughout the winter then reused as-is next spring. The nutrients have been depleted, and all that’s left is a container of sterile potting soil.

It will save time and money to reuse old potting soil, but the growing medium should be properly prepared to support healthy plant development.


How To Prepare Old Soil For Reuse

Start the preparation process by removing the old plants and all visible debris from the soil. Remove bits of plant stems, stringy roots, and dead leaves, then dump the soil into a wheel barrel or similar container where it can remain throughout the winter.

Ensure the potting soil is completely dry before storing it. Damp soil is the ideal environment for mold and mildew to develop and ruin the potting soil you’re trying to save. Dry soil weighs less, so it will be easier to place in storage containers (plastic bags or bins are ideal) and carry for overwinter storage.

Thoroughly wash all the containers with warm soap and water to remove any pathogens or pest eggs attached to the containers. Allow containers to dry and store near the potting soil, so you can find them easily next spring.

Once the old potting soil has completely dried, place it in water-proof storage containers and store it in a cold location during the winter. Exposure to freezing temperatures is good for stored potting soil because it will kill any pests lurking as adults, pupae, or eggs. If stored in a warm location, used potting soil may produce batches of flying and crawling pests next spring.


Solarize Potting Soil

In early spring, it’s time to get out the stored potting soil and finish the preparation process so it will be ready to grow a new batch of plants.

Solarize the old potting soil to kill and lurking pathogens or pests that may have survived the winter. Place the soil in a black plastic trash bag(s) and place the bag in the sun for an hour on a sunny spring day. Temperatures above 120°F (49°C) inside the trash bag will kill most disease pathogens and pests. Once the used potting soil has been heated and cooled, the solarization process is complete, and the used soil is ready to have the nutrients and minerals replenished.


Healthy Soil Mix

Every gardener has their favorite soil amendments and additives, so there’s no right or wrong way to create a healthy soil mix. The old potting soil is sterile and will need to have the nutrients and minerals replenished before it can support new plants.

Creating a 50/50 mix of old and new potting soil is one way to get a healthy growing medium or make a 50/50 mix of old potting soil and compost. The old potting soil can be mixed with organic matter types at a 50/50 ratio to create a healthy soil mix for plants. Check out this article if you want a more detailed recipe to create your own potting soil.

If you have large containers, reuse the old potting soil in the bottom half of the container, sprinkle half a cup of your favorite organic plant food on top, then finish filling the large container with new potting soil.


How To Properly Dispose of Old Soil

There are several options for disposing of old potting soil that is disease and pest-free. A simple way is to sprinkle the old soil around the base of mature trees. While the nutrients and minerals are pretty much gone, the potting soil will replenish any soil that has been eroded around the tree base and slowly decompose and improve soil structure.

* Cover tiny seeds with old potting soil. The sterile soil still contains perlite and humus, making it a lightweight covering for tiny seeds like carrots, radishes, and beets. It also contains moisture-holding properties that promote better germination, but you have to make sure it contains very few weed seeds.

* Make a potato bin. In the fall, create a growing structure that is 3-4 tall and equally as wide, then layer used potting soil, leaves, and whatever other organic material you have. Allow to sit and decompose throughout the winter, then plant seeds potatoes in the bin next spring.

* Make a new raised bed garden using the same layering technique as the potato bin. Create the raised bed, place newspaper, cardboard, etc., on top of the grass inside the bed, then layer old potting soil, fallen leaves, and other organic materials. Allow to decompose throughout the winter, and the new raised bed will be filled with nutrient-rich soil and ready to plant the following spring.

* Work the old potting soil into flower or vegetable gardens. It will add texture to the soil to help prevent compaction, improve soil drainage, and promote a bio-diverse sub-culture under the garden soil.

* Toss it onto the compost pile. As long as the old potting soil is disease and pest-free, it will make a great addition to the compost pile.

* Fill in holes in your garden or landscape. If you have outdoor pets, you probably have few holes in the yard that need to be filled, and your old potting soil got your back.


Use Rain Water

Container-grown plants sometimes develop a white crust on the potting soil surface during the growing season. This is evidence of a salt build-up in the soil and will slow the plant growth and production down.

The salt comes from tap water. Tap water containers salt, and it gets trapped in the potting soil during each watering. As the salt builds up, the white crust forms on top of the potting soil. The salt does not drain out of the container with the excess water, and the plant can not process the excess salt, so it just remains in the soil.

If you capture rainwater to use for watering container-grown plants, it will prevent the soil’s salt build-up. Salty potting soil is not good to reuse. It’s only good for tossing in the compost pile and allowing it to decompose along with other organic material.


Related Questions

How to dispose of diseased soil? In this case, you want to dispose of the soil via the garbage. For harmful diseases, you can also choose to bury the soil deep in your garden (around 2ft – 60cm). Sometimes diseased soil can be composted. However, you have to be a bit careful doing this.

Does potting soil go bad? Dry and properly stored potting soil usually won’t go bad. Check your soil’s smell before using it to ensure no moisture found its way into the container, causing the soil to mold.

How about the soil’s microbiome when re-using old potting soil? Sterilizing also destroys any beneficial microbiome of the soil. It’s best to revitalize your soil by reintroducing beneficial bacteria. Check out this article for a thorough walk-through on how to create the perfect soil