Friday, February 14, 2014

How to Sterilize Potting Soil for Seeds

As an elementary teacher, I always reserve my February vacation as a few days to get the house organized and get those errands done that I've been putting off (or at least a fully organized and functioning home is my lofty goal!).  My 5-year-old daughter and I spent yesterday on our "Total Bathroom Makeover" project with painting the walls, hanging new pictures, and setting up a better laundry system.

One of my enjoyable yearly tasks for February break, however, is starting seeds for my garden.  I LOVE looking through my seed collection and figuring out which ones to start inside.  Our last frost is around April 30th, so I've found that starting "slow" plants (tomatoes and peppers) in mid-February gives them a 12-week head-start, although they can still be started through March.
This year's soon-to-be tomato plants (Beef-Master
 and Early Girl), sweet yellow banana peppers,
and a couple super-hot Orange Scotch Bonnets.

When starting seeds, it's really important to use a sterile potting mix.  Non-sterile mixes invite
fungal and bacterial growth, which can easily cause disease and death to seedlings.  (This happened to me several years ago when I did not use a sterile mix; a white fuzzy fungus developed over the top of the soil and killed my plants.)  While stores carry sterile potting soils, you can also make your own at home for free!

Check the suitability of your soil.  Seeds will not grow well in soil that is too heavy with clay.  To check, squeeze some damp soil in your hand and form it into a ball.  If it breaks apart easily with a light touch, it's OK.  If it stays clumped together, it has too much clay and you should amend the soil with peat moss or compost.  Also, your soil should be free of rocks, twigs, and organic matter that has not decomposed.

Add any amendments before sterilizing.  Compost and manure should be well-rotted.  Peat moss is void of nutrients but can help "lighten" the soil texture.  Do not add fertilizers: they will cause seedlings to grow too leggy, may burn young roots, and should not be used until after transplanting into the garden.

If you are reusing pots or seed flats, they should be cleaned well.  Clay pots can be put in the oven with the soil to sterilize.  Both clay and plastic pots can be soaked in a very mild bleach solution or very hot, soapy water.

Oven Method
Spread soil in glass or metal baking pans or clay pots up to 4" deep.  Insert a meat thermometer into the soil.  Heat at 200 degrees F.  Soil should remain between 180-200 degrees for 30 minutes.  Don't allow soil to go above 200 degrees F or it may produce toxic fumes.  Soil will give off an odor, which is normal.  Cool before using.

Microwave Method
Put about 2 lbs. of damp soil into a plastic microwaveable container (such as a quart-size yogurt container).  Microwave on high for about 2 1/2 minutes.  Don't over-cook as heating too hot can lead to toxic fumes.  Cool before using.

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