sterilizing options

Newnan, GA(Zone 8a)

okay, not wasting my good microwave. I have tons of soil I used last year in pots, and I need an efficient way to sterilize/reuse it, besides putting it in beds. Is the only other option in the oven???
if so, I need to know the particulars

Harrisville, MI(Zone 5b)

tiG,mine goes to the compost pile.makes a great start

Newnan, GA(Zone 8a)

I know, but I dont' have time for that now if I want to use it this year. I won't waste it, I will put in the compost if I have no other options, but it's Promix from pots used last year.

Richmond, KY(Zone 6b)

Frankly, as long as I didn't have any pest or disease problems, I just reuse it. Haven't had any problems doing that yet.

Newnan, GA(Zone 8a)

thanks Brook, I trust you and your experience.

New York City, NY(Zone 6b)

Is sterilizing soil even necessary or helpful?

I ask this question because I am relatively new to gardening, but do understand some things about biology and chemistry.

First, could we define what it is we mean by "sterilizing"?

A. Do we mean selectively eliminating those things in the soil that are unwanted and / or harmful to plants, such as insect eggs, viruses, various bacteria? This is what composting does best.

B. Or, do we mean ACTUALLY sterilizing the soil, which would mean stopping all biological activity in the soil. This second option kills off even helpful bacteria and other organisms.

If we mean Option B, then we need to recognize that we do not actually have in our homes the proper equipment to perform this type of sterilizaton.

Second, what happens when we use home equipment to "sterilize" anything?

A. We heat things. There are two ways to heat something: dry heat or wet heat.

B. Wet heat is more efficient at transmitting heat to the interior of a porous medium (soil) than dry heat...,

C. But, boiling our soil is not really and option ;~).

D. So we are left with dry heat. To me, this means either my stove oven or my microwave oven.

How hot can we make these things? And how hot do we NEED to make it to sterilize our soil?

Well, the answer is..., we can get our stove ovens up to about 600 F, and the temperatures actually reached inside our microwave ovens are difficult to determine, as we can't put a thermometer containing metal into them. At 600F you will turn your soil to ash.

Soil is, in general, moist, so will conduct heat well. However, to actually kill off many sorts of micro~organisms the heat, wet or dry, must be applied under pressure. This is why laboratories and hospitals use autoclaves: they apply heat under the pressure of from 10 ~ 40 atmospheres.

Now, my wife who knows all about viruses, bacteria, etc, says that the odds of killing off these things with our home equipment is very limited. In fact, what she actually said was that heating the moist soil would in all likelyhood provide a nice dark, warm, moist environment to increase the populations of the very things we are trying to kill off.

She said the compost pile was probably a better idea, as in the pile these organisms would lose the contest for nutrition with the beneficial organisms, while simultaneously maintaining the biological diversity of the soil.


This message was edited Wednesday, Mar 12th 12:29 PM

Hillsboro, OH(Zone 6a)

tiG, I have been watering with water mixed with peroxide for all my seedlings and rooted cuttings. I have seen very little of pests and no molds, fungus or moss type growths. I guess this is fine on a small scale, don't know how big of an amount your are speaking. I love to reuse soil and see what seed surprises pop up. LOL

Newnan, GA(Zone 8a)

did you use old soil to start seeds? the only thing I worry about and won't do is use my last years tomato soil for brugs. even though nothing had a virus or anything just don't want to chance it.

Hillsboro, OH(Zone 6a)

No, I don't use old soil to start seeds (always buy new seed mix). I use it to pot up other stuff. Once I prick out seedlings, I throw all the extra dirt into a big pot or bucket and I use it outside for potting or transplanting. That is how the surprise seedlings finally sprout. :)

Murfreesboro, TN(Zone 7a)

Ditto what Badseed said. When I'm being really good and organized, I have three waste bins in my GH:

one for trash (plastic stuff, etc.),
one for organic stuff that goes to the compost bin (cuttings, discarded seedlings, etc.); and
one for used soilless mix that I put into containers, or to pot up hardier stuff :)

(When I'm in a hurry/less organized, I have one bin for trash and one for compost, and that's where the discarded mix goes, until guilt sets in, which usually coincides with opening yet another 40-pound bag of soilless mix. Then I rummage around for another waste bin, and get religious about saving it. Otherwise, I find I can burn through several bags of mix in short order - ouch!)

I don't reuse the mix to start new seedlings, but I've found it's perfectly fine for the other uses mentioned.

rural, WY(Zone 3a)

Another idea, drench soil with water repeatedly to wash out excess fertilizers and chemicals. They build up and you probably want to start fresh. Peat is acidic and many critters can't take on it.

Henderson, NV

Pour boiling water through it.

I did that with several pots this year and all plants are doing fine.

This message was edited Monday, May 12th 8:38 PM

North Saanich, Canada

We have been sterilizing our soil in the microwave, with no problems so far. Our dirt pile ended up on the ground, over top of some horesetail, which of course grew right through. I don't want horsetail all over the palce, so we are sterilizing the dirt and only using it in pots and containers. We usually add some water to it, and run the microwave about 25 minutes. It comes out steaming hot, and after cooling, we just carry on!!


Newburgh, IN(Zone 6a)

Badseed, what is the ratio of peroxide to water that you use to water seedlings?

Some of what I planted last year got moldy and did not make it to transplant size.


mulege, Mexico

I was just reading about seed starting at the www.emarerica. com site. They first soak seed in a week peroxide solution, then in a weak EM solution, then in pure water.

They were talking more about sprouting seeds for eating sprouts but I think if you continue to water with a weak EM solution after planting the seeds, that keeps a probiotic situation going.

I have to read more about this but I noticed this thread so I thought I'd jump in.


Newburgh, IN(Zone 6a)

What is EM solution? The link does not work for me. Thanks

Cincinnati, OH(Zone 6a)

I googled it. EM= effective microorganisms. Looks like another company trying to sell another expensive unnecessary product. I didn't read much but didn't see much to impress me.

What is it that you're trying to sterilize out of the used potting mix? Disease? Weeds?


Cincinnati, OH(Zone 6a)

Oops- I forgot the link.

mulege, Mexico

Thank you, Karen. I mistyped the name.

They have some videos that are interesting. A lot of the uses are off this threads topic. It has a lot of uses.


Newburgh, IN(Zone 6a)

Still looking for the ratio of peroxide to water. Anyone know? Thanks

mulege, Mexico

Use 1-2 tsp. of HP in 2 quarts of water. Soak for 20 minutes.

Newburgh, IN(Zone 6a)

Thank you katiebear. :-)

Magna, UT(Zone 7a)

Badseed, what is you peroxide/ water ratio? I have a tiny bit of mold on some of my pots. I have sterilized soil that I bought. I don't trust to just pot right in it. I use the oven method. I and my sister in law had problems with potting soil having fungus and gnats of some sort in it.

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