Seed starting time again, that time of year comes around pretty quickly. The days are getting milder, snow melting, the sun warmer and the bulbs are peeking up through the snow.
I reuse soil all the time, mix in some old with some new and pot up plants I have divided in the garden. But I won't reuse old soil for seeds and seedlings and I am even careful if I have a half used bag of potting soil left over from last year. Up until a few years ago, you could purchase a fungicide called Damp-Off that you added to your water when watering in your seeds and watering your seedlings that prevented the damping off fungus that devastated your newly emerged seedlings. That is no longer on the market and there is no equivalent product that can be used, not even organic (please tell me if you know).
There are antidotal remedies, hydrogen peroxide, chamomile tea (probably puts them to sleep), cinnamon (at least it makes the soil smell good) and powdered charcoal, that are 'supposed to' prevent the dampening off. Water your seedlings from the bottom (good advice) and give them good air circulation (more good advice), but the best advice is to use good sterilized potting mix. Most mixes are pre sterilized, but what if you have leftover potting soil from last year. But remember seed has become expensive.
There are a few ways of sterilizing your soil, all involve cooking (technically). Get out your measuring spoons and scales. You can use a pressure cooker, steamer, oven (every reference says this will absolutely stink up your house) or your microwave.
I used the microwave method last year with really good results (at least no damping off) so I'll try it again. This method is all over the Internet, it must have come from one source and I have not seen anybody saying it doesn't work.
Add enough water to the soil so that when you squeeze the soil, a bit of water comes out.
Place 2 lbs of this mix in a Ziploc (or equivalent) bag, or deadicated microwaveable container and place it in the microwave with the top open. Make sure it is stable. The last thing you need is to clean potting soil out of the microwave.
All references I found said to microwave for 2.5 minutes / 2 lb. My scientific mind questioned this since everything I read referring to the oven method was to maintain the soil at 180 F for 30 minutes. Even if you are microwaving, should not the soil temp hold at 30 min also? Does it even reach that temperature? Soil temperatures should reach at least 180 F but not more than 190 F. Many references refer to toxicity to the seeds if the soil goes above 200 F.
I have a soil thermometer, very useful for checking the garden soil in the spring and extremely useful in this application. After 2.5 minutes I resealed the bag with clothes pins and the thermometer and took a reading.
The temperature of the soil only went to 160 F. Time to do a few more experiments. I microwaved a new batch (room temp) for 3 minutes. It just barely reached 180 F. One more time. 3.5 minutes brought the soil to 188 F but the soil temperature dropped below 180 within 7 minutes.
I'm going to use it anyway, because I am quite confident in my soil mix and this was just extra insurance. If I was at all concerned, I would buy a small bag just for seeding or do more experimenting to get it right.
Next time I need this soil I may try 4 minutes and try to insulate the bag by wrapping it in a towel or something similar to keep the temperature up for a longer period. Just a thought.
Now a tip for seeding.
Those containers that mushrooms come in, great seeding trays.
All you need to do is poke a few holes in the bottom and they are recyclable too.