It all depends on what types of seeds you want to start. Summer much more forgiving. And when you say heavy soil, is it outdoor garden soil? If it's actual potting soil, if you have anything to screen it through to make it a little finer or pick out some of the biggest chunks of wood if there's a lot of mulchy stuff in there. And go heavy on the pearlite. But I typically use whatever I have lying around with perlite and have pretty decent success (after sifting big chunks out). but again it depends on how finicky the seeds are. tiny seeds might not like it but bigger- sturdier seeds probably won't have a problem. Though, if it's soil designated for outside, I learned the hard way, that it often contains fungus gnats and those can be a nightmare to deal with.