Sterilizing seed starting soil
Posted on Feb 28, 2009 under Compost, Gardening, Self Sufficiency | No Comment
These past few weeks we have had snow and freezing nights, but also a couple of very sunny days that warmed our greenhouse up to 30 degrees. Spring is just around the corner, it’s still 3 weeks before it officially starts on March 20th but we’re close enough to start planning for planting. April and May are the preferred sowing months for most vegetable crops when growing outside, but we can start a lot earlier in the greenhouse.
Last year we had quite some weeds growing in our seed trays. Some where so badly infested that it was impossible to tell the weeds from the planted seeds, so I decided to try to sterilize the compost this time. After a little digging and sifting in our compost heap I got more than enough material to mix some seed starting soil. I mixed in about 30% peat and 5% coarse sand.
To sterilize the mixture it should be baked in the oven, the first try didn’t work out too well though. After sitting in the oven for well over 4 hours the center of my soil mixture was still pretty cold. This shows the great insulating capability of soil, a trade that is much appreciated by people building earth sheltered homes. What I did wrong was compacting the soil in the deep oven tray by pressing it down with my hands. By doing this I could fit al the soil I had mixed into the tray, wrongly assuming that this would be the way to sterilize double the amount of soil in the same time. The compacted soil, however, has a greater mass per volume unit and thus has a far better insulating and heat absorbing capability.
The mixed soil was far more than I needed anyways, so I gave it another try today. This time I made sure the soil in the baking tray was nice and loose. The soil should be a little damp and covered tightly with aluminum foil. A meat or candy thermometer will show you the temperature in the center of the soil. With the oven set to 90C (195F) It took about 4 hours for the soil to completely heat up to about 85C (185F) Maintain this temperature for another 30 minutes and the soil sterilization should be all done.
Be careful not to let the soil heat up beyond 95C (200F) or it will form toxins that will not be appreciated by your plants. Allow the soil to cool down and keep the foil on until you are ready to use it.
