baked dirt?

October 29, 2005 8:22 PM

Ive had some problems with mould killing my house plants so Id like to sterilize the dirt from an old pot before i plant some thing new. Can I just bake the dirt or will that hurt the nutrients/good microbes? Other ways to sterilize dirt?

I suppose the better question would be how to stop/prevent the mould in the first place - aside from moving away from the PNW.

posted by Tryptophan-5ht to home & garden (9 comments total)

They do sell sterile dirt at the garden store and hardware store. It's cheap as dirt!
posted by Miko at 8:29 PM on October 29, 2005

I have worked with plants in an academic biology lab. We bake our dirt in an autoclave. The plants still get mold from time to time (from other sources besides just the dirt). It doesn't really hurt the nutrients. And can't good microbes be added later?
posted by muddgirl at 8:31 PM on October 29, 2005

You might also want to consider washing the pot out as stuff will stick to that too.
posted by pwb503 at 8:51 PM on October 29, 2005

Boil it in a pressure cooker.
posted by delmoi at 10:22 PM on October 29, 2005

miko - not as cheap and "free dirt i already have"
posted by Tryptophan-5ht at 11:20 PM on October 29, 2005

I'm going to second muddgirl. I also used to work in a plant lab, and we autoclaved our dirt all the time. We used to add inorganic fertilizer to help plant growth. Any microorganisms that the plants need will be floating around in the air (as is the mold you're having trouble with) or already on the seeds/roots
I found this info on soil sterilization for the home gardener. To sterilize your pots scrub them in a bleach solution.
Now, as for preventing the mold in the first place: improve the air circulation around your plants, make sure the top layer of soil dries out before you water, and make sure the plants are getting enough light. Mold in plants is usually caused when there is too much water sitting around.
Is the mold on the actual plants, or just on the soil?
posted by nprigoda at 3:38 AM on October 30, 2005

Ha! My soil geochemistry class comes in handy!

The real nutrients coming from soil are basic elements: nitrogen, sodium, iron, magnesium, sulfur, etc. The only effect that baking has on these is, well, none. It will remove the water and any heat-sensitive microbes sticking around, including your mold. Bake away! Just be sure to add plenty of water, because the baking will dry out the soil to a ridiculous degree (all of the moisture left in air-dry soil is already unavailable to plants, so baking makes it even more so. This isn't a detriment to the future soil plant inhabitants, though, as long as you water). Maybe consider using distilled water, also.

Tacking on to nprigoda's point, are you using pots with drainage holes on the bottom? That would prevent long-term buildups caused by watering past the saturation point, and maybe help with the mold problem.

As an aside:
I was under the impression that microbes and things are generally not necessary for proper plant growth, and a quick search of the academic literature has only really pointed out cases of negative microbes (here for example) but I could be wrong (nprigoda? muddgirl? I would be interested in hearing about this if you know anything)
posted by The Michael The at 5:46 AM on October 30, 2005

may I horn in? For the opposite problem, has anyone tried this stuff? I was thinking of ordering some as a gift for a green-thumbed relative, but I can't tell if it's legit.
posted by small_ruminant at 9:24 AM on October 30, 2005

From my plant path days - are you sure it's mold? There are many different kinds of plant pathogens with different solutions depending on the microorganism.

Spores are incredibly hardy; in some species, even autoclaving will not kill all of them. OTOH, depending on the plant pathogen, industrial-agricultural decontamination practices include covering (and outdoor area) the dirt with dark sheet plastic and let the sun heat the soil for a few months.

Are you growing the plants from seeds, or are you seeing mold developing (at the base of the plant, in the roots, in the stem, or on leaves) on mature plants?

If you're growing from seed, you can soak them briefly in 10% bleach (rinse well afterwards) before germinating them.

If you're seeing microbial growth in roots/base of the plant, maybe you're overwatering.

If you're seeing growth on stem/leaf, you can mist with 10% bleach (or 75% isopropanol/ethanol), wipe-down, and rinse with water.

The easiest solution may be, as others have mentioned, scrub down your pots in bleach and buy new soil ("I'm better than dirt! Well, most kinds of dirt, not that fancy store dirt...").
posted by PurplePorpoise at 9:51 AM on October 30, 2005