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Don’t try to sterilize your soil in a microwave. In fact, don’t sterilize your soil at all.

  • Author: Jeff Lowenfels
    | Alaska gardening
  • Updated: February 18
  • Published February 18

Seed-starting season always, and I mean without fail, brings questions about using sterile soil to avoid damping off diseases when germinating our seeds for the summer. It is time for Alaska gardeners, actually all gardeners, to stop even considering sterile soil.

I write books on soil microbial activity. I am writing another right now. It just slays me when I read a fellow gardener tell another that sterile soil is the only way to go. It is not.

My first memories of sterile soil come from my dad. He had a big wooden contraption that sterilized soil, one bucket at a time. He would toss in some of his carefully made compost and turn it on. Twenty-four hours later, we would dig out the warm soil and use it to start seeds.

I inherited that machine. It did not make the trip up here and I am pretty sure it ended up in a garage sale, and no one knew what it was to be used for.

Anyhow, like most gardeners of the age, I had no idea that fungi — Rhizoctonia spp. and Fusarium spp. and Pythium spp. — are the most common pathogens responsible for damping off. All I knew was seeds needed sterile soil or a soil-less mix like perlite, vermiculite or peat — which may or may not be sterile, but no one knew that in 1970.

So, for the first couple of years of starting seeds here in Anchorage, I used the microwave. Right here I have to interrupt myself and strongly insist that regardless of whether I persuade you to use soil full of bacteria and fungi when starting seed, do not use your microwave. My kids, 40 years older, can still describe the smell of cooking worms, nematodes and other soil food-web members.

Why use living soil when planting seeds? The microbes in it are almost always diverse enough to be able to keep the fungi that cause damping off in check. These are in the soil or actually transported on the seed itself, including bacteria that attach themselves to the roots of the seedling and use it for nutrients.

It is not a bad idea to use clean growing containers and fresh, living soil, which can be infected by these fungi if you have ever been hit with damping off disease. Any household bleach should do the trick on containers. And, it is always a good idea to ensure your seedlings and more mature plants are in an area with good air circulation.

In fact, some things I start in the same soil I grew the same kind of plant in last year. Tomatoes, begonias, pelargoniums, cannabis and fuchsias, for example, started right in the large container they will grow in all summer, have the advantage of the exudates and the microbes produced last year. It gives these new plants a head start.

As for your soil, you can test it with a few seeds now. If they die back, get some new soil. We have plenty of time yet, not that you should put off a test.

Then there is the rhizophagy cycle — write those words down and remember you heard them first here. This is a new model for yet another way plants obtain nutrients. In this instance, they actually let bacteria into their roots and mine them for nutrients — and more, as you will be learning over the next few months.

One study showed up to 30% of a plant’s nitrogen is obtained as a result of rhizophagy. If you use sterile soil, this does not happen until the microbial herd is established. In the meantime, seedlings can suffer symptoms that look like damping off. In actuality, without the rhizophagy microbes, seedlings don’t form root hairs and this makes it extremely unlikely the plant will survive. Who knew?

So, please trust me. Don’t use sterile soil. You want those microbes. And, by the way, don’t sterilize your seeds with hydrogen peroxide or anything else. Like the soil you want to use, they should be covered with bacteria that help them germinate and develop.

Jeff’s Alaska garden calendar

Alaska Botanical Garden: Sign up for camp. Join. Sign up for the virtual spring symposium and visit the virtual symposium vendors. Details at alaskabg.org

Stored plants: Fuchsia, begonias, Rhodochitin, pelargoniums. Time to get them out of storage and up and running. More next week.

Water plants: My greenhouse fan goes on when the sun is out. This is a sure sign that plants are up and running, and you need to water them a bit more often. Check them daily until you can establish a routine that has adjusted to having light again.


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