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Build Your Own Steam Soil Sterilizer
by John Harmon
January 26, 2003

The good news about last weekend is that we finally got enough snow to provide a good insulating blanket on the ground and just in time too! Whitehorse airport reported 6 cm of snow on Friday and after a day without snow Sunday came in with a whopping 13.5 cm at the end of the day. This is the second heaviest one day January snowfall recorded at the airport since 1943. The snow was desperately needed and with temperatures dropping to -40 outside of town mid-week was just in time to help protect plants from cold damage.

It's hard to think about spring with all the cold and snow but it's time to plant geraniums and some varieties of begonia. Before you plant make sure your soil is sterile to avoid problems later. Sterilizing soil is always a hot issue for gardeners in the north because we start so many of our plants indoors as we get ready for spring. Nothing is more disturbing than losing a batch of plants to some virus or a bug that hatched from the soil you used not to mention the "damping off" group of fungi. Sterilizing your soil or soiless mix before seeding will avoid all the problems and disappointment.

Seedlings are usually started in commercially available potting soil, which consists of some combination of loam, peat moss, perlite and vermiculite. Such mixtures are usually labeled "sterilized" or "pasteurized," indicating that they have been heat-treated to kill insects, weed seeds, and disease-causing organisms. Don’t believe everything you read on a label! Even potting soil labeled as “sterilized” may not always be. I got some potting soil last year that claimed it was sterile but got an infestation of fungus gnats from using it as well as problems with damping-off. It’s best to buy the good stuff like "Terra-Lite Redi-earth" which is a fine textured soiless mix perfect for seedlings and it’s cheaper than buying small bags of potting soil.

If you don’t want to use chemicals and you’re not sure of the validity of the claims on the bag of potting soil you can sterilize your own mixes. Any mix that you use to start seedlings like soiless mixes, potting soil or garden soil, store bought or homemade, can be sterilized. You can heat the soil a number of ways. Small batches can be done in the oven in a roasting pan or in the microwave. If the plastic container of soil in the microwave melts down you will know it got too hot! The easiest way to sterilize soil is in a soil steamer. Commercial store-bought steamers are many hundreds of dollars but you can make your own for much less.

You will need a metal container about two feet square by two feet high. You can build one or Duncans Limited on Copper road will build you this box with a lid out of 24 gauge galvanized steel for about $100.00 plus tax. This box will hold enough soil for most gardeners to sterilize at one time and will last for many years. You don't have to worry about fumes from the galvanized metal because you won't be getting it hot enough to burn the coating off.

Next you will need a hotplate. Any old electric hotplate will do as long as it will boil a pan of water, which is the next piece you will need. I use an ordinary baking pan. Just make sure it's not "Teflon" coated! It must be big enough to hold at least a half-gallon of water.

Place the hotplate and pan in the bottom and punch a hole for the cord from the hotplate to come out of. Leave about four inches of space above the pan to put in the wire mesh. Drill some holes at the proper height all the way around the box and put in some three eight's inch bolts long enough to stick into the box an inch or more. These will support the wire mesh. Cut a piece of half inch hardware cloth to fit snugly inside the box supported by the bolts. Use a sturdy gauge of hardware cloth to support the weight of your damp soil. To keep the soil from falling through, add a layer of metal window screen on top of the wire support mesh.

Now all you need to do is fill the pan with water, put the wire in place with the screen, fill it to the top with damp soil or soiless mix, put on the lid and plug it in. The boiling water creates steam that will rise up and heat your mix until it reaches the right temperature. A regular household meat thermometer will let you know when the soil gets to 180 degrees F. That temperature will kill insect eggs and larvae, some weed seeds, and the damping-off fungi. At higher temperatures beneficial organisms are killed and dissolved salts are released in the soil which may be toxic to plants so keep an eye on it so it doesn't get too hot.

With your homemade electric steamer you will have a safe sterile mix to start your plants in and not have to worry about something in the soil killing them off. All you have to do then is keep your seedlings warm until spring and you will have healthy happy plants ready to set out when all this new snow finally melts.


John Harmon owns and operates Tropicals North. Write to John at The Real Dirt, c\o 211 Wood St., Whitehorse, YT., Y1A 2E4 or e-mail tropnorth@polarcom.com.


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