What do you do with old soil?

Santa Cruz, CA(Zone 9a)

When I replace the soil in my houseplants, I never know what to do with the old soil. I have heard a school of thought that it shouldn't be dumped in the garden or compost pile because of the potential for harboring unwanted stuff e.g. bugs, diseases, etc. But it seems like such a waste to just throw it out. Does anybody know much about this issue?

Port Huron, MI(Zone 5b)

I can't agree more about throwing away old soil..but I'd also like to know about sterilizing soil if anyone knows how, instead of having to buy sterile soil

I was hoping that we'd have a good answer by now, but since we don't, I'll throw in my two cents.

As I understand it, you can bake the soil to remove any diseases, weed seeds, etc. I have an oven specifically for baking dirt, and a couple hours at 140 degrees has worked consistently for me in sterilizing my medium.

Also, I have a compost tumbler, which easily gets over 150 degrees and maintains that temperature for 3-4 days.

With either of these two methods, I'd feel very comfortable re-using potting soil.

Having said all this - I don't re-use my soil. I just toss it into the back pasture where it mixes with the grass. I get my soil in giant bags, 10 bags at a time, and the cost of operating the oven just doesn't outweigh the cost of new soil.

My two cents! I'd love to hear from someone more knowledgeable on the subject.


Murfreesboro, TN(Zone 7a)

Unless the plant was obviously diseased or bug infested, I have always thrown mine in any bed that's mostly clay, I figure can't make it worse :) and will probably help.

Now that I have compost piles, I usually toss it in there.

You can sterilize soil in the microwave, although the quantities are small.....let me see if I can find the directions and cooking time.

San Lorenzo, CA

Where I live, the county provides each household with a green waste container that's picked up at the same time as the garbage. That's where I put stuff like used soil (like unsuccessful seed-startings) that I'm afraid to add to my own compost. Somewhere, there's a monstrous county-wide compost pile!

I usually empty the container on the lawn and investigate the soil for grubs especially vine weevils, after that I have always used it as a mulch on the beds or in the compost pile once all the weeds have been extracted. Unless as Go Vols has said the plants are obviously diseased, I've never seen a problem with it.

Port Huron, MI(Zone 5b)

Thanks all...I'll just keep putting it on m compost

Springfield, MO(Zone 6a)

Look for info on sterilzing your own soil here:

(The 6th one is from the Master Gardener Manual)

Lyles, TN

If your compost heats like it should, the heat kills all those pathogens. Turning useless byproducts into something useful again is why we make compost. You just got to tend your pile, and have faith in yourself ! :)

This message was edited Sunday, Jun 3rd 3:43 PM

Uh, ya'll don't throw rocks here or anything do you?

Unless I have a very contagious item or issue I totally reuse each and every grain of soil. When I get root mealies or some nastiness like that I toss it all in the trash, but container gardening on a balcony is tough enough with lugging bags of soil through your bedroom every month.

So, I mix my own to each specific need, and 'use it up, or wear it out'.


Lyles, TN

Cena~ Did I sound too harsh? Didn't mean it that way.:)

Wewahitchka, FL(Zone 8b)

I put mine into the compost tumbler. Like Dave said, it gets pretty hot inside there.

Deep South Coastal, TX(Zone 10a)

Hi all, I do like Karma and Vols, I use it if it didn't have a disease. We have clay and it can use all the help it can get. If it had something wrong, I compost it under black plastic. Pythium scares me, so I will toss the soil if it had that. I haven't had any problems and I've been doing it for years.

Oh Ivey, NO! I'm just a paranoid soul... and have had rocks thrown before. I hang out with a better crowd now!!!

I did try baking my soil once before, but all the little bits of tree/wood/bark caught on fire and smoldered for HOURS (more like days), so, ... I'm not allowed to put dirt in the oven anymore. I had to beat the smokealarm with a mop to get it to shut UP! Already!

I really like that black plastic idea... just wait until I get my own yard. I have 300 plants now!!! I can take over the world then...

Um, see why they tend to throw rocks? ?


Lyles, TN

Cena~ You wouldn't happen to have a picture of you beating the smoke alarm with a mop, would you? (chuckle) I have actually heated soil in a shallow pan on the woodstove, STINKS! But that image really made me laugh. Thanks.....Ivey

Santa Cruz, CA(Zone 9a)

My fiance already thinks I'm a nutty plantoholic--I will probably be institutionalized if he sees me baking dirt :) But knowing I can just throw it in the yard is good enough--just didn't want to waste it.

(Zone 6a)

This is the first year I have saved the potting soil from my wife's numerous hanging flower baskets. I simply piled it up in a back corner of my yard and planted a tomato plant in it.......I have 91 tomato plants this summer.....guess which one is the tallest, greenest, sturdiest! I am also trying container gardening this year for the first time, using an assortment of 2 gallon pots up to about 7 gallon.....using Shultz potting soil....2 cu. ft. bags from WalMart for $7.44 a bag. So far, looks like I will never go back to planting in the ground again! I have 36 different varieties of "heirloom" tomatoes planted....and they all love the containers....but the potting soil quality is the real key I think....loose and well drained.

SW, WI(Zone 4b)

I always throw mine outside, in spots where the animals have 'nested' to sleep, dug holes! or in places where I've dug up bulbs, plants, etc, or in places where the soil needs 'amendment'....I've never had any problems...of course if the soil obviously had disease/bugs/fungus I wouldn't do so.

Hughesville, MO(Zone 5a)

Ours is either reused immediately if the plant was healthy or thrown under the cages of the 2 bunnies who live outside. The poultry have free access to it and do a lot of scratching in it. After just a couple of weeks I feel free to reuse it anywhere. If the plant in it was diseased, I may throw the soil in the big compost bin or just on the pasture.
I have 'cooked' soil in the mirowave. I got it fairly wet, then heated it until it steamed well. I have to admit it didn't smell very good.

Santa Barbara, CA

I either compost the old potting mix or bury in garden.

Let me make a distinction between sterilization and pasteurization. Planting media are never sterile but their are either heated/steamed to at least 140F long enough to kill off fungi and disease organisms. Life still exists, often in "seed" forms. A fully sterile soil could be susceptible to massive explosions of the first colonizing microbes -- there are no buffering/balancing microbes naturally present in growing medium. There are transplant growers who innoculate their freshly pasteurized potting media with specific fungi and other microbes before starting their crop.

Noblesville, IN(Zone 5a)

I throw mine in a large pot with some manure and run soap water (Ivory) through it and plant. I am not saying that everyone should do this but that I do and it works okay for me.

Travis AFB, CA(Zone 9a)

I dump all my containers' soil into the big garden just work it in with a garden fork. The only "problem" I've had is violets, alyssum, and lobelia pop up everywhere!

The last pot to be emptied was a 16" pot with moss rose and blue salvia in it. I emptied it where I'm going to try to start a lasagna garden. I wanted the moss rose seed to be ready.

I never thought about pest and diseases; but all the bugs in my pots are in the gardens, too.

Reynoldsburg, OH(Zone 5b)

I read somewhere that you can poor peroxide into the pot and stir it around. I don't know if it works. I am going to try it this year.

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