Answer: To some extent this will depend on what you are starting. In general the medium should hold both air and moisture so that the seeds do not rot before they can germinate. The mix should also have a structure conducive to quick rooting. Its structure should make it wettable to the dampness of a wrung out sponge. Fertility, interestingly enough, is not particularly important until the seedlings put on some size. Most annuals and vegetables seem to start just fine in the relatively sterile peat based soilless mixes which come with perlite or vermiculite added for drainage and aeration. Some perennials, trees, and shrubs, however, would prefer more of a sandy mix and some would prefer say a more acid or alkaline mix. Germination temperature and stratification requirements can also come into play and are at least as important as the soil mix itself.
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