Just before we got married, my husband and I were starting our first garden in western Washington. AJ was working in a greenhouse at his college and he had all he could want in the way of spent soil and compost. He wanted to save money and use it for starting our seeds instead of buying commercial starting mixes. I was leery of plant diseases since the compost pile hadn’t been maintained at a temperature to kill them. He assured me any diseased plants didn’t go into the pile. I was still resistant to the idea but he nagged and whined and poked until I finally decided he could have his way.
When he brought the soil home I noticed right away that there was a very healthy population of millipedes or centipedes, I don’t remember the difference now, the itty bitty ones were what we had. I did some internet research and couldn’t find anything about them eating seeds or seedlings so I grudgingly used the soil.
Until I knew something was wrong because something should have sprouted by then.
Well my friends, those little demons do eat seeds. Most veraciously, and I’m very sure they would have happily munched on seedlings too. They would chew through a weak spot in the seed coat, usually were the radical (seed root) would emerge from and eat the seed from the inside out.
So, I got creative. I figured compost piles get pretty hot and heating the soil would kill the bugs. I decided to try it since the worst case scenario I could think of was totally killing all the soil bacteria.
This year I find myself with a place to have a garden that was totally unexpected, and broke. So very broke, that I’m once again making my own starting mix.
How to Sanitize Soil – or Using free soil to start seeds
~300F for 15 minutes, add more time if soil is very wet or using deep dishes.
Picking your starting soil
The best thing you can use is finished compost, otherwise a sandy soil will work fine, you will just need to feed your starts more or add compost to the soil. By “finished compost” I mean decomposed manure, bedding, plant matter that smells like soil and has a nice deep color and rich smell. If it’s not finished it will be chunky and smelly.
Starting with a soil that contains a lot of clay would probably turn into a disaster. Clay is the worst to try to start seeds in and heating it is liable to make weird pseudo-pottery. If you have a very dense soil and a little money buy some perlite or vermiculite to add to it; it will make the soil fluffier and a little bit goes a long way.
Pick out large rocks, sticks and any worms or carabids (big black, sometimes iridescent, super beneficial beetles) and return them to the great outdoors to bless your garden later.
Preheat your oven to 300F. A lower temperature would probably work, but the numbers below 300 are rubbed off my toaster oven and at 300 I’ve still had a few critters come out alive.
Poor your heated soil into a clean food grade bucket (we don’t want any BPA in our soil, now do we?) or box without lots of holes. You can let it cool all the way before you use it or just enough that it’s not steaming anymore and you can comfortably run your hand through it.
If you are trying to kill a specific disease you should look for information about what temperatures kill it and adjust the process if needed.
That’s all there is to it folks, plant those seeds to your hearts content!