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I am planning to do some indoor gardening. I've build some containers large enough to host 1200kg of soil.

I am gonna use the soil form the garden and forest, but I would like to sterilise it first. I am not interested in commercially available sterilised soil.

Firstly, when is soil considered sterilised? By which I mean to what temperature must be heated and how long must stay at that T.

Secondly, what procedure do you recommend? I am actually dealing with large amount of soil, so I am wondering if there are any commercially available products for that kind of work and

Thirdly, how it is done on industrial scale?

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It will take 30 minutes at 180 to 200 degrees F. minimum. You will need additives. The best potting soils have very little actual soil in the mixture. You'll need vermiculite, do not use peat, decomposed organic matter, mycorrhizae fungus, bacteria added to your soil (soil should not exceed 25% of the total volume), do not use sand. After mixing everything except the fungus and bacteria to add later... THEN bake it for half an hour. I should be nice to tell you that the smell is worse than burning BEANS. And lingers. You will find that commercial potting soil is better and easier and less expensive. Why do you not want commercial potting soil? I am very particular and very distrustful of commercial anything. This is one of the few things that can't be cheapened and ruined or made to be a detriment. We always learn mostly by mistakes anyway. Just write down what you are doing, how much of any additive, what the soil was comprised of (gotta have a soil test well would be beneficial), what the pH was, what water you are using, fertilizer regimen, what kind of mulch and how decomposed it was, the kind of pot, no rocks or gravel beneath the soil, what species plants you plan to use in that pot, how much light is available and does this fit with the plant's needs, with pH in particular. Raise the bottom of the pot off any surface to enhance drainage. If I thought I could make better soil for less or even equal, I would. But I KNOW this stuff and I would never plant pots without commercial potting soil...unless apocalypse happens. I HAVE tried making my own soil, never again. Too lazy, doesn't save money and the stupid commercial stuff works GREAT. Need a big oven or you'll only be able to do a gallon at a time. Industrial ovens, flash heat, bacterial killing lighting, air pumps to pull the air out and pumps to replace the air.

  • Thanks for stopping by. Great tips. You mentioned mycorrhizae fungus.. These are naturally occurring in forest soil, right? But if I add forest soil after the sterilization, than I guess I've done nothing. What about bacteria? What source do you recommend? I will be planting tomatoes and peppers. I haven't done any soil tests and I do not have a plan for fertilizing yet. My goal is to make it as natural as possible, so that basically means leave plants take care of them selves as much as possible. But that also means that the startup conditions of my soil is optimal. – sanjihan Apr 20 '17 at 8:12
  • Stormy, the best potting soil has loam, as in JI mixes - even though its sterilized, its still soil, and that loam may be up to half in some mixtures, so I don't agree with your third sentence. – Bamboo Apr 20 '17 at 10:05
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Generally, ordinary soil is not sterilized commercially because its not necessary. Bagged or loose topsoil can be purchased, but it is not intended for use in pots, so there's no need for it to be sterile. Sterilized potting composts are available for sale, usually produced using an aerobic hot composting method because most contain humus rich materials. Sterilized loam is used in some potting mixes, but I'm not at all sure how manufacturers carry out the sterilization process - likely in industrial machines which produce steam and heat.

If you insist on doing this rather than the better option of buying good potting soil, you will need to sterilize the soil you collect in relatively small batches - some methods involving microwaves, steam or the oven are listed in the link here - note if you use the oven method, you will need a sugar or meat thermometer to check the temperature of the soil as it heats so it does not get too high: Tips To Sterilize Potting Soil, Garden Soil And Soil For Seeds

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I've sterilised small amounts of homemade compost (or at least I believe I have, I was mostly interested in killing weed seeds) using the following method:

  • Fill a metal container with the compost.
  • Leave in the sun, protected from any rain, for a few days to dry out a bit (optional). Note that this step alone will deal with some things as it probably reaches more than 40°C and for hours at a time.
  • Heat. I was using an old biscuit tin holding a couple of litres, and what I did was to shut it in a kettle barbecue (with the lid on the biscuit tin) after I'd finished cooking. The coals were still burning brightly when I did this, then I left it until the next morning.
  • Rehydrate. It was as dry as dust.

This certainly killed all my weed seeds to use it as a seed-raising compost. Germination was as good as you'd expect from bought stuff. This was outdoors so inherently well ventilated.

I pot up tomato plants etc. in unsterilised garden compost (usually with some blood and bone added) whether I'm growing them in the greenhouse or outdoors (and feed with tomato feed later). Note that I'm not chasing every last scrap of yield; I'd rather reduce the amount of stuff I use.

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