How to Sterilize Soil

Posted by Katie Elzer-Peters
April 14th, 2009
Filed in Soil and Fertilizer
Tags: Compost, Soil and Fertilizer

Soil sterilization is a great technique for organic gardeners to master. For potting soil, you will need a sterile mix to avoid bringing weeds and pathogens into your indoor plants. You may also wish to sterilize outside soil if you wish to start over on a patch of garden. Here is why you might want to sterilize soil, how you can do it and how you build up a healthy population of beneficial organisms after you sterilize the soil.

Why Sterilize Soil?

Soil is full of living things: bacteria, fungi, worms and insects.  Many of the organisms in soil help break down larger pieces of organic matter like grass, twigs and bark into small particles of humus, which is the best substance you can have in the soil. These organisms also break down nutrients into forms that plants can more easily take up through their roots.  That’s the good part about the living things in soil.

The bad part about living things in soil is that there can be detrimental fungi, bacteria and insects in the soil along with the beneficials.  In most garden soil, there is also an abundance of weed seeds.  If you are gardening organically, from start to finish, and need to create a new flower bed, one way to rid the bed of weeds is to sterilize the soil.  Doing this can save you lots of time and money in the long run.  If you are starting your own seeds, you need sterile potting mix so that seedlings do not succumb to damping off, and other life-ending diseases that plague seedlings grown in un-sterile potting mix.

How to Sterilize Potting Soil

Sterilizing soil for seed starting is one of the easiest things you can do to make seed sprouting a success.  Here’s how you do it:

1. Put your desired amount of soil in an oven roasting bag
2. Add some water to the soil
3. Tie the bag
4. Poke a hole for a meat thermometer
5. Put the soil bag on the pan
6. Heat the soil to between 160-170 degrees.  (Set the oven on 200, and lower if temp. of soil goes above 170).
7. Use the meat thermometer to check the soil temperature.
8. Leave it in for about 30 minutes
9. Remove soil and let it cool thoroughly before planting anything in it!

If you want a recipe for making your own potting soil mix, be sure to sign up for our free report!  We include several recipes

Solarization Sterilizing Garden Soil

Garden soil is a bit more difficult to sterilize, but it can be done using a process called solarization.  This is a method of using heat from the sun to kill disease organisms that cause plant problems like verticillium wilt, root rot, damping off and others.  In order for your garden to have the full benefit of solarization, the soil needs to reach a temperature of 114 degrees F (46 degrees C) for at least four to six weeks.  Here’s how to solarize your soil:

1. Rototill the soil in the area you want to solarize.
2. Water the area so that the top foot is moist.
3. Cover the area with clear plastic, held down with bricks or tacks.
4. Leave in place for at least a month.  In cooler areas, leave in place for two months.
5. Check the soil every two weeks to make sure it is still moist.
6. Remove plastic and plant the garden!

A few notes about sterilizing garden soil:  For the most part, beneficial organisms will survive the solarization.  You can add humic acid or compost to the soil to put back in the good stuff after you’ve solarized.  It is not a bad idea to test the compost for weed seeds by watering a little and seeing if it grows.  You can water with compost tea to add beneficials back as well.

Solarization goes most quickly in the south, during sunny, dry days.  For almost every location, it works best in the summer.  If you solarize in the summer, try to leave the soil covered with black plastic during the winter, or plant a thick green mulch crop to build up fertility, and keep weed seeds out until you plant the following spring.  (Always remove the plastic before planting!)

Happy gardening!  Bloom day is tomorrow.  Yippee!