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Frugal Seed Starting Mix

By Dutchlady1
February 17, 2013

You can buy a 'soilless mix' to start seeds, but doing so can be expensive. It is perfectly OK to 'sterilize' your potting soil yourself in a regular oven. Be careful! Do not let it get too hot - 30 minutes at 180 to 200 degrees is ideal.

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Name: Pegi Putnam
Norwalk, Ca. zone 10b
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Samigal
Feb 16, 2013 6:30 PM CST
I knew you coould sterilize your gardening soil in the oven, but never tried it. Didn't know just how long and what temp. to "cook" it at. Thanks for the information. Group hug
Name: Paul
Utah (Zone 5b)
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Paul2032
Feb 17, 2013 7:21 AM CST
I pour boiling water through potting soil to sterilize it. Also if I have had the soil for some time and it is very dry the hot water will wet it. I let it cool down before potting the seed.
Paul Smith Pleasant Grove, Utah
Name: Renée
Northern KY
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KyWoods
Feb 17, 2013 11:38 AM CST
I live in an apartment--would the oven method send an odd smell through the building?
Name: Arif Masud
Alpha Centauri (Zone 9a)
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KAMasud
Feb 17, 2013 1:04 PM CST
Yes, it will. The organic components will bake. Though, I don't think it would be objectionable. If not then boiling water is best.
Regards,
Arif.
Name: woofie
NE WA (Zone 5a)
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woofie
Feb 17, 2013 2:18 PM CST
You can also do this in the microwave, which is sort of like what Paul does. You add water to the soil (just enough to dampen it) and microwave it long enough to boil the amount of water you added. Leave it in there until it's had a chance to cool, because it's HOT!
Sterilizing soil in the oven definitely creates an odor. DH hates the smell, which is why I looked into using the microwave.
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Name: Hetty
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Dutchlady1
Feb 17, 2013 5:27 PM CST
Yes, it does smell funky but you can always say you'll use the warm oven to bake something in it afterwards Smiling which will quickly dissipate the smell!
Name: woofie
NE WA (Zone 5a)
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woofie
Feb 17, 2013 8:50 PM CST
But then I'd have to actually COOK something! Blinking Hilarious!
Confidence is that feeling you have right before you do something really stupid.
Name: Arif Masud
Alpha Centauri (Zone 9a)
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KAMasud
Feb 17, 2013 9:20 PM CST
I would not like to bake or cook after baking cow patties Blinking . Not because of germs, but the next item cooked may get that baked earth/patties smell Thumbs down .
Regards,
Arif.
Name: Renée
Northern KY
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KyWoods
Feb 18, 2013 6:41 PM CST
Eeeew....we need a barfing smiley. Rolling on the floor laughing
Name: woofie
NE WA (Zone 5a)
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woofie
Feb 18, 2013 7:03 PM CST
The oven IS a good way to sterilize larger amounts of soil, tho. I use turkey roasting pans when I want to do a bunch at once.....but only if I'm not planning on eating anything very soon! Hilarious!

I've read that you do need to keep an eye on the temperature of the soil. It needs to be HELD at around 180 for half an hour. (Get out the old meat thermometer!) Apparently you can cook it TOO long and it does nasty things to the soil. Can make it toxic to plants, somehow. And you do need to make sure it's moist when you start. About the level of moisture you'd use if planting seeds in it.
Confidence is that feeling you have right before you do something really stupid.
Name: Arif Masud
Alpha Centauri (Zone 9a)
Native Plants and Wildflowers I helped beta test the Garden Planting Calendar Container Gardener Plant Lover: Loves 'em all! Enjoys or suffers hot summers Multi-Region Gardener
KAMasud
Feb 18, 2013 9:24 PM CST
Silcia, Silica, melts and flows beyond a certain temperature.
I have baked soil but never used the oven afterwards in a hurry and told my better half to give it a warm up time in order to dissipate the earthy smell. Should see her face though, not to happy. Terracotta ovens, you should have those.
Regards,
Arif.
Name: Rick Corey
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RickCorey
Feb 19, 2013 8:16 PM CST
I like potting soil that drains faster than my yard soil, and lets more air in. I should say "drains better than my AMENDED outdoor soil, because even my best soil in raised beds is 30-60% clay.

I consider pine bark mulch or nuggets "frugal enough" for a few 5 gallon buckets, and I mix in some % of "whatever" commercial peaty mix, or baled peat I'm willing to splurge for.

Lowes has very clean small bark nuggets for $4.20 per 2 cubic feet. I don't recommend Home Depot bark mulch because my local HD sells really dirty log-yard trash for $3.50 / 2 cubic feet. YMMV.

I screen the bark, to remove too-big nuggets and too-fine mulch fibers and powder. Big bark chunks make great mulch, or you can grind them smaller and re-screen them. Fine bark dust can be mixed into a raised bed just fine, just don't make it 25% bark all at once.

$ 4.20 for 2 cubic feet means 28 cents / gallon, or a 5-gallon bucket for $1.40. I think the bucket cost more than that.

If I want bark as an amendment to speed up drainage and aeration, like a substitute for coarse Perlite, I want bark nuggets or chips around 2.5 to 5 mm or 1/8" to 3/16". A little coarser than gritty.

If they are long bark chips or shreds (not spherical), I would like them up to 3/4" long and 2-3 mm in small dimensions.

If I want just water-retaining bark fibers and shreds, as a base for a soil-less mix, I start with the cleanest mulch I can find and afford, with as little dust and powder as possible. Then I try to screen OUT as much under 1-2 mm as I can, which is difficult.

This really fine stuff clogs up a mix and tends to exclude air the same way too much peat does. Instead of lots of really fine bark, you would probably be better off with a little peat or coir.

Right now the smallest screen I have is 1/4". Probably a 1/8" or 10 mesh screen would be better for working with mulch that includes fines and powder. Al uses some 1/16 window screening he found to "de-dust" the bark fines.

Anyway, to hold much water, you probably need a lot of the bark to be smaller than 3/32", 1/10", or 2.5 mm. Or instead, add a little peat or coir, or "whatever" commercial potting mix you have.

Of course, that's soil-less and nutrient-free, so you have to water with soluble fertilizer or start with bark plus compost and feed it things like fish fertilizer.

Others might prefer to consider the bark a "stretcher" and "aeration improver" instead of the base of the mix. Add 20% to 60% gritty or coarse bark, 1/16" to 3/16" (or even some 1/4") to whatever you were using, and knock your costs down by 15-40%. At 28 cents per gallon, it's cheaper than almost anything except what you can shovel from your own yard.

Big chunks like 1/4" or larger in effect reduce the volume of your pot, since roots can't penetrate it.

The finer the bark is, the less peaty mix you need. If your bark nuggets are mostly over 2 mm or 3/32", you need a fair amount of peaty mix or it won't hold much water, and you might have to water it more than once per day.

If you have a timer and a drip system and were thinking "hydroponics", I think fast-draining bark might be second only to expanded shale or clay nuggets. But I haven't tried that yet.

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