Potting media sterilization

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Potting media sterilization

by Grey » Wed Dec 29, 2010 11:59 am

Hello everyone.

I'm looking forward to Spring and like everyone have had my fair share of wins and loses with my first Winter and dormancy period. I'm planning ahead and making preparations for the Spring re-potting event and thought I'd enquire regarding something I've been curious about for a while now.

I've had various problems with mould, fungus and bugs and although I know not all are bad for my plants, I want to do what I can to ensure their safety and keep their potting media as "clean" as possible. I'm wondering about sterilization. I have a few questions and I hope you don't mind me asking.

My biggest concern is mould and fungus. I'm severely asthmatic and have attacks when near a non-carnivorous plant whose soil has mould or some form of fungus (I know it'd be the same with carnivorous plants, too). I know regular repotting may aid with this problem, as do fungicides, however I would like do to everything I can to prevent (or slow down) the need for these measures. I don't honestly think regular root disturbance is a good thing.

1a) My main question is: Will sterilizing potting media affect the pH of it or will it cause the media to lose its acidity?
1b) If not, will sterilizing the soil help keep my plants fit and healthy?

2a) If so, what do you use for sterilization?
2b) If you use something like a microwave or oven, do you add water? How much? How long do you cook/microwave it and what temperature/wattage do use? We have an 850W microwave if that's of any help.

The potting media(s) I use are as follows:
For my smaller carnivorous plants (dionaea, pinguicula, drosera & utricularia). I add a little perlite for my sarracenia:
Klasmann medium grade sod cut Irish moss peat with elutriated granite sand at a ratio of 4:1
For my nepenthes:
Klansmann fine grade Irish moss peat, granite sand & orchid bark

I'd like to thank you for your time, and wish you a wonderful New Year!

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Re: Potting media sterilization

by BradR » Wed Dec 29, 2010 4:28 pm

Hi Grey:

The only way to truely sterilize potting mix is in a pressure cooker or autoclave. You need temps higher than the boiling point of water to kill all organisms and spores. The problem is that it accelerates the breakdown of the media (bad) and makes it a perfect culture medium for the next round of fungi. It will quickly be colonized again by airborne spores. Sphagnum and peat are great media because they are acidic and low in nutrients. They also have beneficial organisms already established. This tends to inhibit pathogenic organisms and favors beneficial organisms as long as conditions are aerobic. When you read about sterilizing soil in the oven (at 180 F) or microwave, what they are really doing is pasteurizing the soil. This reduces bacterial counts and kills all the molds and most pathogenic fungi. (Molds are killed above 160 F). But it also kills the beneficial organisms present in the peat.

So I don't sterilize the peat based media for routine CP use. I have done oven pasteurization for seed starting soil and soil used for plants just removed from tissue culture. I also bought some mycorrhizial inoculant to soak soil in after the pasteurizing. The idea is to add back the beneficial organisms before the bad ones get established. The one I'm using is called Great White.
http://www.plant-success.com/index.php/ ... hizae.html

Good Luck,
Charles Darwin: "The Venus Flytrap is one of the most wonderful plants in the world."

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Dionae, Grey, Matt

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Re: Potting media sterilization

by Grey » Wed Dec 29, 2010 4:41 pm

Hello Brad,

Thank you for the information, you have been most helpful. I'll have a look at the link you gave me and will do more research as well. I'll refrain from doing anything until I know more, and may simply flush the media or drench it for a couple of days in the hope it'll remove any insect eggs without destroying helpful bacteria. I'll also look into purchasing a fungicide for emergencies, too.

Thank you!

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Re: Potting media sterilization

by Adam » Sat Jan 08, 2011 5:38 am

It's my new favourite way of growing VFTs from seed. Boil the madia, wait for it to cool, put into freshly washed little pot, place seeds on top, put little pot into freshly washed jar, add water to the bottom of jar, seal jar and leave like that for 6 months.
Is your VFT experiencing these conditions?


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