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Can You Re-Use Potting Soil From Your Containers?

Q. Mike: My wife likes to plant flowers and vegetables in large pots. Come winter, we empty the pots. The used potting soil usually contains an extensive root structure and often is beginning to sprout weeds. What is the best way to store and recondition the potting soil for reuse the following year? Thanks!

    ---Dr. Mitch; The Children's Hospital of Philadelphia

Both my wife and I thoroughly enjoy your programme for the sound, useful advice and the humorous and boisterous tone in which it is delivered. Would you please settle a long-standing disagreement between us: Can one re-use potting soil after having grown plants/flowers in it for one season? The husband says: "Yes! You bet-your-potting-soil!" The dear wife (who, admittedly has spent 100 hours gardening for every hour the husband has dabbled) claims that such soil is spent after one growing season and may now harbour insects and/or disease. Who is right? Please do let us know…

    ---Michael (the husband) in Spokane, Washington

Q. An excellent question, and one that deserves an intelligent discussion. But instead of that, I'll tell you what I do with mine.

First, let's review the basics of container contents. Because you are going to trap these poor plants in a finite space, as opposed to the great outdoors where they can send their roots out much further in search of help, you have to supply them with a light growing mediumthat drains exceptionally well. That means no garden soil in the mix. Instead, the ideal medium for containers is three-quarters soil-free mix and one-quarter compost.

"Soil-free mix" is the term I use for high-quality potting soil; it may also be called professional mix, seed-starting mix, sterile growing medium or some other synonym. It's generally composed of milled peat moss (with a little lime to adjust the pH), perlite and/or vermiculite (naturally occurring minerals that are 'popped' in big ovens) and some compost or {quote} "composted forest products". Some packagers substitute coir (shredded coconut fiber) for the peat; and some companies add nutrients to the mix (which is bad if the nutrients are man-made chemicals, but wonderful if they're natural things like worm castings).

Mix one of those mixes up with some high-quality compost, and you'll have a growing medium that retains moisture and drains well, contains a nice amount of organic matter, and is light enough for you to move the containers around fairly easily.

So—what do you then do with this wonderful stuff at the end of its first season? Because the soil will expand and contract greatly over a harsh winter, those who grow where the ground freezes hard should empty out plastic, ceramic and clay pots to protect them from cracking. Or you can just bring the whole schmageggie inside to a place that will remain above freezing. (If you do empty them out in the fall, remove any roots or weeds and add them to your compost pile. If you store the pots full, plan to remove this debris when you freshen up the mix the following Spring.)

In my opinion and physical reality, the only hard-core issue of re-use here is The Tomato Rule. Potting soil that was used to grow tomatoes should not be used to grow tomatoes the following two years. BUT that soil can be used to grow flowers, bush beans, peppers, salad greens—whatever you want, as long as it's not tamatas. Conversely (like the sneakers), soil that hasn't ever been used for tomatoes (or that hasn't seen their roots for a few seasons) can be used to grow this year's love apples.

One way to achieve this noble end is to have two big galvanized or hard plastic trash cans, label one with a T and one without, and use these to store your soils over winter. Don't worry about otherwise mixing the soil from different pots; I actually prefer to combine mine to mitigate any potential nutrient imbalances and such.

The following season, buy some fresh soil-free mix and use it to freshen up every pot that gets filled with old soil. How much? Up to a third new mix if your old soil is really old or if it seems to be bulking up on you; less if your old stuff is still light and fluffy. Always add fresh compost to the tune of one-quarter of the container.

Now the risks. Insect carry-over is fairly remote, as is the risk of keeping a disease alive other than the soil-borne wilts that attack tomatoes. Weeds could be an issue, especially if you don't mulch the tops of your containers with shredded leaves (which I highly recommend as the leaves also retain moisture, a very important consideration for pots in direct sun or during an especially hot dry summer).

But those weeds (and any tricky diseases) will still be much less of an issue than in outdoor gardens, and the weeds can be even further avoided by layering the new season's compost a couple of inches thick on top of the old soil-free mix instead of mixing it in.

And if, like me, you garden in ground and in containers, it's a wonderful idea to give one or two of your containers a completely fresh set of clothes every few years and mix their old potting soil into your garden, where its mix of lightweight ingredients will be welcomed by the roots of your plants—especially if those poor rooties have to try and fight their way through the misery of clay.

Read these Previous Questions of the Week on CONTAINER GROWING BASICS and GROWING TOMATOES IN CONTAINERS for more info on those important topics.

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