In containers, you want potting mix. Some use soil but it surprises me that it works. If you read labels, many brads that say "potting soil" actually contain no soil. Most are peat based, some contain wood, perlite, vermiculite.

Pro Mix and Miracle Grow seem to be preferred by most wintersowers.

Karen

You want mix, not soil. Soil will turn to a brick of concrete, and mix will stay light and water-absorbent.

This was my 1st year & I went with Trudi on this one and used the same soil that I use in my raised beds outside. It's 1 bag of top soil, 1/2 bag of manure and 1/3 bag of black satin mulch. It worked fine for me but I'll probably mix it up some this year and see if it makes any difference to use Miracle Grow. I know that some plants I won't use anything but the cheap stuff because they did so well in it this year. Marigolds, Chives, Morning Glories, Cosmos, Basil and 4 O'clocks would probably grow in concrete so why spend that extra $$$ on Miracle Grow?

Strouper

Also a first time wintersower this year. I tried some with regular topsoil, some with a mix of topsoil and peat and vermiculite and some "soil-less."
The topsoil did turn into a brick BUT some of my most successful seedlings were in that batch (larkspur, chrysanthemum, clasping coneflower)
I had most consistent sprouting with the mix of soil,peat and vermiculite. I had a lot of really healthy sprouts from these and they are filling my beds so well now that I will probably only wintersow annuals from now on.
As for the seeds that I had in the soil-less mix: almost none of them sprouted but I did get a nice crop of algae on top.
I know everyone else says soil-less is the way to go, but it wasn't good for me.
All the best with it. I had lots of fun with it thanks to the generosity (seeds) and advice from people on this forum.

George