I successfully created several composts over the last two years using, what I believe is the "cold compost method". I have an open heap which I turn from time to time during the initial year. I used it in the second year for all kind of things mainly for enriching the soil, compost tea and for a seed-starting-mix (I even used it pure). In my eyes it works well in these three kinds of applications.

However there is one thing which I'd like to improve. Especially for the seed-starting-process I noticed that there is plenty unwanted germination most likely coming from unwanted seeds in the compost.

Is there a way to clear, posterior, the compost from weed-seeds?

I read somewhere that putting the soil into the microwave helps. With that I assume that the applied heat will kill at least the seeds and maybe other unwanted "things". Can I use an oven as well? Is cooking the compost-soil over an open fire a possibility? What would be the highest temperature allowed? In other words, is there a risk of reducing the quality of the compost when it gets too hot?


2 Answers 2


Yes, there is a possibility of reducing the quality of the compost if it gets too hot. The optimum temperature range for production of compost is between 57 and 65 degrees C, which is enough to destroy seeds (with the possible/occasional exception of tomato seeds) and various pathogens. Much more than that and you destroy everything, rendering your compost completely inert. If you can find a method of detecting how hot your compost is getting if you, as you suggest, heat it over a fire or put it in the microwave, then fine, but I can't think how you would do that in practice. It would be better to produce your compost using an aerobic, hot system in the first place, which would mean the end result is humus rich with a good level of bio diversity, although I realise that would be a lot of work, given the average temperatures where you live.


In my mind, the effects of the application of heat would be similar to that of what you'd expect to see from cooking your food. Some chemical reactions will speed up, others will halt. When you cook a potato, you convert starches to sugars. When you blanch green beans, you stop the enzymes from converting sugar to starch. So it seems that cooking compost would reduce starches to sugars and make the result easier for microbes to break down.

Although it seems to be much more work to sterilze the compost than to remove the weeds as they germinate, I don't see a problem with cooking the compost from a soil-amendment point of view. The composting process is actually an oxidative reduction of plant material similar to burning, with the end result being humus, which is resistant to further degradation for 100s of years. Humus, being organic, will of course burn (ie oxidize), but cooking is not burning. I think the humus would be stable enough at high temps so long as it's below burning temps.

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