How to Reuse Potting Mix Safely

by Corinne Mossati

Previously, I detailed how to amend soil at the start of the growing season. In this article, I explain how to reuse potting mix and save on arguably the most expensive part of gardening: potting mix.

Reuse Potting Mix

Potted Garden – Photo © The Gourmantic Garden

One of the tasks I’ve set myself when I started edible gardening as a hobby was to be mindful of costs, to plan well, to reuse, upcycle and minimise as much as waste as I can. To date, my biggest spend aside from the initial set up of the raised garden beds has been buying quality potting mix and as tiny as my courtyard garden is, I seem to need an endless supply.

As I have explained before, the majority of my plants are in containers, the reason being, the border around my garden is made of poor quality soil that’s about 5 to 30 cm deep sitting on concrete.

One of the biggest mistakes I’ve made in my first year of gardening was to buy the cheapest soil I could get and I’ve paid the price, literally, with stunted growth, a waste of time and a lot of frustration.

But all was not lost as there were major lessons to be learnt. You may be wondering, can you reuse the potting mix in your containers or raised planters? The short answer is yes you can, provided the plant that was grown in it was disease free. Here are the steps I use for how to reuse potting mix safely.

How to Reuse Potting Mix Safely

Reuse Potting Mix

Repurposed Council Recycling Boxes – Photo © The Gourmantic Garden

1. Make sure the soil had plants that were disease free otherwise you could be transmitting pathogens to your new plant. Don’t reuse or compost diseased soil.

2. Using a garden fork, break up the soil, aerate it and remove remnants of any roots. Some schools of thought says to keep them in to decompose but I’ve found unwanted shallots growing from old roots in a pot. It’s really up to you. If I’m about to sow carrots for example, I want the soil to be free of obstacles so I will dig around and remove old roots. I usually tip the used potting mix into a recycling crate if not required immediately then amend the soil later. If you really want to go the extra mile, you can sift the soil. I do that occasionally to try and save as much soil as possible instead of throwing out a clump with attached roots.

3. Add organic compost, like mushroom compost or your own garden-made, worm castings, aged cow or sheep manure, a sprinkle of blood and bone, and a slow release fertiliser if you wish. Then add fresh potting mix. The ratio varies depending what was growing before, whether it was a hungry plant like tomato and what will you plan on growing next.

4. Mix it all in thoroughly, water it well, add mulch (I use hessian sacs) and leave it to cure for about week or two before planting.

I’ve had success with this approach which meant minimising waste and saving money in the process.

Other Ways of Reusing Old Soil

Reuse Potting Mix

Potted Rose Geranium – Photo © The Gourmantic Garden

  1. Use the spent soil as the bottom layer when setting up a new raised garden bed.
  2. Compost the old potting mix if you use one.
  3. Add it to your garden to fill a space.

If you grew diseased plants previously and you’re not keen on disposing of it, you could pasteurise your potting mix by bagging it in black garbage bags and leaving it to bake under a hot sun (check the weather forecast). I have read that you can microwave it or bake it in your oven at 100C for half an hour but there’s no way I’m bringing that mess into my kitchen!

By following simple steps, you can safely reuse potting mix, save your money while practising sustainable gardening.

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