You Can Put Old Potting Soil in Compost – Here’s How!

Bags of potting soil are expensive, and it feels like such a waste to throw it away after only one growing season. While it is necessary to replace the soil in your pots and containers seasonally, there are many ways to use spent potting soil, including adding it to your compost heap.

Old potting soil is a great addition to a compost heap. You must first sterilize the potting soil by leaving it in the sun inside a black plastic bag for four to six weeks. This will kill any plant pathogens and unwanted seeds in the soil, preventing them from contaminating the compost pile. 

Composting is a fantastic way to nourish the soil in your garden, allowing your plants to thrive. In early winter, when I empty the pots and containers to store them away until spring, I always add the spent potting soil to my compost heap. This puts the soil to good use for another season, and you will get high-quality compost to use throughout your garden.

Why is it Necessary to Replace Potting Soil?

Many experienced gardeners recommend that one empties growing containers in early winter. In areas where it frosts, wet soil in pots expands, causing pots to crack. This task is not just important to avoid damage to your terracotta – there are more important reasons why it is crucial to replace the potting soil in all your pots and containers seasonally:

• Whether you use organic or regular potting soil, by the end of the growing season, the soil will be nutrient-depleted, and your pots will need to be emptied and refilled with fresh potting mix. A bag of potting contains a limited amount of nutrients, which plants use up as they grow. As soil is exposed to the elements, it weathers and minerals leach out of the soil. By the time plants have finished fruiting or flowering, there will be no more nutrients left in the soil.
• Diseases or pests that affect plants remain in the soil, making it dangerous to re-use, as it will transmit pathogens to new plants. 
• If you have had pots containing the same soil for a year or two, you would perhaps have noticed the hard, white crust that forms on the soil surface. This is due to salt build-up from watering plants with tap water. Regularly replenish the soil in containers and try to water with rainwater as much as possible. 

Use Old Potting Soil in your Compost

Potting soil is an expensive investment, especially if you buy premium organic potting mix to fill your growing containers. To throw it away at the end of the season would be a waste. It is better to look for ways to safely re-use it.  

One way to re-use spent potting soil is to add it to your compost bin or heap. This will naturally replenish the nutrients in the soil so that it can be used to grow more plants in the next season. 

Sterilize Soil Before Adding it to the Compost!

Even if plants did not show any signs of disease, there might be some type of pathogen or pest in the soil. Old potting soil also usually contains unwanted seeds from invasive species. 

To avoid contaminating your compost and potentially spreading diseases and weeds throughout your garden, one must sterilize old potting soil before you add it to compost.

There are a few simple ways you can do this:

• Empty the soil into black plastic bags. Loosely twist the bags closed and leave them in a hot, sunny spot. Let them bake in the sun for four to six weeks. This simulates the heat inside a large, active compost heap. The heat kills pathogens, weed seeds, and pests. Although this process kills the beneficial soil bacteria, you can replenish the microbiome by composting the sterilized soil. 
• If you live in an area with freezing winters, leave the bags of spent potting soil out over the cold months. The intense cold will also kill off pathogens and pests. Some cold tolerant unwanted seeds may survive, but at least this method lowers the risk of plant-harming diseases. 
• For smaller quantities of potting soil, one can bake the soil in the oven. This method gets rid of any pests, pathogens, and seeds. Spread the soil in a thin layer on an oven tray, cover it with foil, poke ventilation holes, and bake at 200 degrees F for half an hour.

How to Compost Old Potting Soil

How you add your old potting mix to your compost will depend on how your compost system works. Some people use bins to compost in, while others construct bays for larger compost heaps. 

Whichever way you compost, there is one general rule to follow: add equal volumes of green matter and brown matter. Green matter is high in nitrogen, while brown matter is high in carbon. This ratio makes the perfect general-purpose compost.

Old potting soil is brown matter. So are dried leaves, straw, coffee grounds and cardboard. Green matter is manure, vegetable clippings and grass clippings.

Add old potting soil, manure, leaves, grass clippings, vegetable scraps and coffee grounds to your composting bin or pile in layers, ensuring you have an equal amount of green and brown matter, by weight, not by volume! 

Water the compost heap. Put a plastic tarp over the top of a compost heap to keep the rain off and the heat in. Weigh it down with bricks or stones. 

It will take four to six months for all the components to decompose, break down, and the compost to form. Make your compost heap in fall when there are plenty of leaves to gather, let the magic happen over winter, and shovel out your wonderfully rich, homemade compost in the spring!

Conclusion

Adding old potting soil to a compost heap or bin, along with manure, grass clippings, dried leaves, coffee grounds, and vegetable scraps, is a fantastic way to re-use the soil. 

Potting mix is expensive to buy, and to throw it away after one growing season when it is nutrient-depleted, is wasteful.

Replenish the nutrients in the potting soil by composting it. This will feed the soil microorganisms, which in turn will feed your plants.

To avoid contaminating your compost heap or bin with pathogens or weed seeds, sun bake old potting soil in black plastic bags for up to six weeks before using it in the compost heap.

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