Steriliseing soil

It is actually soil from molehills that I am referring to there are quite a number of weeds germinating in the soil I have spread out in a tray, sitting in the greenhouse and as I don't have several months to wait for them all to germinate before making up my baskets, I use soil and compost in them as soil holds the moisture longer than compost, also I don't want to use moisture retaining gell, the growing medium in the baskets will be reused for various things next year and though they are said to be safe, I really don't want to use them