What Should You Do with Old Potting Soil? Here's When to Toss It or Reuse It

In the garden, renewal reigns—from dormant perennials emerging in spring to shrubs bouncing back from a severe pruning, nature refreshes itself. Does the same apply to potting soil?

Depending on how many potted plants you have or the size of your containers, potting soil can be a lofty investment at the beginning of the season. The cost of filling a large pot with high-quality soil may rival the price of the plants that will fill it. So it's only understandable that you wonder whether you should reuse the potting soil to cut down on cost and spare yourself the trouble of lugging big bags of soil every spring, then lifting the tangle of soil and roots out of the pot in the fall.

Read on to find out if it's a good idea to simply reuse the soil from your potted plants next year.

What Makes Good Potting Soil

A great garden begins with great soil. The same adage applies to container gardening. Here is what potting soil does for plants as they grow.

Nutrient Content

Potting soil is the sole supplier of nutrients to the annuals, perennials, shrubs, and small trees that you plant in the confined space of a container for six months or more. The nutrients contained in the soil mix, along with those that you add in the form of fertilizer, deliver life-giving nutrition to the plants throughout the season and ensure foliage health, flower production, and overall growth. However, these nutrients are finite. Once they are absorbed by the plants, they are no longer available.

Structure

Fresh, loose potting soil provides the basic structure in which plants develop roots. In potting soil, just like in garden soil, nutrients, water, and oxygen can only be accessed by plants if there is sufficient pore space between the soil particles.

Pore space creates essential tunnels for nutrients, water, and oxygen to move in the soil, further fortifying the roots and boosting plant growth. Available pore space is diminished when roots fill it or the soil gets compressed as the soil particles break down over time and pack together. This limits oxygen and water access, stunting plant growth.

Good soil structure also plays an essential role in plant support. The roots anchor in the soil, providing a firm foundation for stems, branches, and leaves.

A Single-Season Star

Even the best potting soil is usually spent at the end of a single growing season. After months of vigorous growth, regular watering has leached out any soil nutrients that were not absorbed by plant roots. A robust container garden likely has a dense root network supporting the lush foliage. That dense root network has filled the soil profile with root mass, leaving little soil available for new plants to take hold. In short, the potting soil is ready to retire. It has fulfilled its job and has little left to support another season of plant life.

After the first hard frost presents you with a collection of bedraggled container plants, it’s time for fall clean-up. The compost pile is the best destination for healthy plants and the potting soil in which the plants were grown. However, do not compost plant debris and soil of plants that had pest and disease issues, but dispose of it in the trash.

To speed up the decomposition process in the compost pile, chop the plants into small pieces and tear apart the soil mass. Layer the potting soil and plant material with dried leaves to further promote decomposition. Water the pile gently, or simply wait for rain to jump-start the process of turning organic debris into nutrient-rich humus.

Money-Saving Potting Soil Ideas

While it's best not to reuse potting soil, here are a few ways you can stretch potting soil and reduce the cost of filling your containers every year.

50/50 mix. To reduce container gardening costs, some university horticulturists support the idea of filling a container with a 50/50 mix of old and new potting soil. Only recycle soil from healthy plants and sift through it to remove all plant debris. Thoroughly combine the old soil and new soil to create a consistent mixture.

Pot filler. Reduce overall soil needs in a large container by filling the bottom of the pot with empty milk jugs or aluminum cans. Spread landscape fabric over the recycled containers to keep soil from filtering into the space. Top the landscape fabric with at least 12 inches of soil to provide ample rooting space for plants.

Buy in bulk. If you are filling many containers with potting soil, consider purchasing in bulk. Check with your local garden center about bulk pricing. You may need to transport it yourself or accept a driveway delivery, but the cost savings might offset additional work required to get the soil on site.

Steer Clear of Garden Soil

It's tempting to consider using garden soil instead of costly potting soil when filling containers. While garden soil is right for growing plants in a traditional in-ground setting, it's much too dense for container gardening. The amount of space between garden soil particles is so minuscule that container plants struggle to send their roots into it and extract nutrients from it. Potting soil is a worthy annual investment if you want thriving container plants.