Is it Safe to Reuse Potting Soil or Should I Start Fresh?

Can I Reuse Potting Mix?

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Every new gardener who grows in containers has this question:

Is it safe to reuse my potting mix or do I need to start fresh?

Here’s the quick answers:

• Best Choice: Start fresh each year.
• Second Best: Replace half and add fertilizer.

Yes, potting mix can be expensive, heavy to transport, and it seems wasteful to replace it after a single growing season but fresh really is best and sometimes safer.

Here’s why.

Potting mix degrades as plants use up nutrients. The good stuff is either used up or washed away.

Container plants can also develop various plant or soil diseases.

As summer goes on, you’ve probably seen how the potting mix in a hanging basket of flowering annuals changes.

The roots gradually fill the basket, nutrients are used up, and the potting mix become compacted.

In some cases, even if you wanted to reuse the soil you couldn’t because it’s basically a glorified root ball.

On the other hand, a larger container of shallow-rooted herbs may still have some soil life left in it.

So, it also depends on what you grew, the condition of the soil, and if it’s disease-free.

And yes, we’ve all reused potting mix without any big problems but in general it’s less than optimal.

The Half-New Solution

If you don’t want to start fresh, a compromise solution is:

• one part (good) old mix
• one-part new mix
• a serving of slow-release organic fertilizer to make up for lost nutrients (read your product label).
• and (bonus) one-part compost 

We never use soil straight from the garden in planters is because it lacks the traits needed for successful container growing.

How to Save Old Potting Mix

For me, the idea of sending potting mix or plastic pots to landfill just seems too wasteful and unnecessary.

If I am confident old potting mix is simply depleted but disease-free, I just toss any loose soil in my garden beds.

In rare cases, there may be a lot of loose soil leftover and I’ll remove any plant bits and keep it in a container with a lid (nice and dry) for next time.

If it’s mixed with plant matter (roots, stems, leaves, etc.), I chop the whole thing up and add it to the compost pile.

A proper compost pile naturally heats up. Once it reaches temperatures around 160°F (71°C), unwanted bacteria and pathogens are killed off.

Another option I have not tried is to bake potting mix in the oven (180°F | 82°C for 30 minutes). This never seemed realistic to me due to the volumes of potting mix I use, but it is an option for smaller amounts including houseplant potting mix for reuse.

Related: How to Make Worm Tower for In-Ground Composting

POTTING MIX TIPS 

• Read potting mix labels.
• Get to know the ingredients.
• Watch for unusual additives.
• If you grow edibles (veggies, herbs, fruits, nuts) in containers, be sure the product is food safe.

Organic Potting Mix

What Should I do with my Planters at the End of the Growing Season?

If you keep containers filled over the winter—even weather-resistant ones—unwanted bacteria, viruses, fungi, and pathogens can grow in the soil and harm your plants next season.

So, you need to empty them in fall.

Yes, it’s more work. But, if you’ve ever had disease spread to new plants, you know what a heartbreak that is.

Tomato plants are infamous disease-carriers. Many annual flowers have fewer problems.

Fall Routine

• Empty containers, wash, and disinfect with 1-part bleach and 9 parts water.
• Dispose of any diseased plants and their potting mix.
• Add remaining plant materials and organic potting mix to compost pile or store soil in dry containers.

Spring Routine

• Assuming they were cleaned and disinfected in fall, the pots should be good to go.
• When choosing new potting mix, get a type designed for the plants you are growing.
• I use an organic potting mix that is marked ‘food safe’ intended for edible plants (fruits, veggies, herbs) for all my container growing since everything is going in the compost pile later and I don’t want synthetic fertilizers or other soil additives ending up in there.

Organic Potting Mix

• If you’ve set some old potting mix aside, and you’re sure it’s healthy (no signs of mold or other weird growth), use that too along with a serving of granular organic fertilizer (follow instructions on product label).
• You can further improve your potting mix with the addition of good compost (decomposed organic matter) if it has been properly prepared. The natural heat of a compost pile can destroy unwanted bacteria and pathogens.
• Mix your potting mix ingredients thoroughly to provide a good growing medium.

~Melissa the Empress of Dirt ♛

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