Karen

Because the best time to secure your mix is now, I don't know what to tell you. Maybe take some of the stuff as it is right now, put it in a few WS containers, and check out its drainage (a milk jug should have maybe 5 large, 1/4" holes). If you have some potting mix, do a container with it so you can compare how potting mix acts next to your mystery stuff. Soak the mystery stuff and the potting mix and see how well they drain. If your stuff turns into a muddy slurry, that's not so good.

Do you have any peat moss or perlite? You can make your own mix with that and the stuff you just had delivered. I save money by getting peat and perlite in bulk, and mixing it with a little garden soil (mine is sandy) and either composted manure or compost. I also dump used potting mix into there. My mix is mostly peat and perlite, because that's what makes drainage and looseness. So, if this stuff of yours doesn't drain well or is too dense, you still have the option of using some of it in a homemade mix. Or you can just go to the store and buy some potting mix. And like kcqrna mentioned, it's MIX you'd want to buy. Don't buy potting soil. Depending on how much you want to WS this year, it may be worthwhile for you to make your own mix if you want to do a lot, and like projects. Or call around to actual nurseries as well as big-box stores, and find out the largest sizes of potting mixes they sell. Some mixes like Fafard's and Metro Mix come in huge bales that are very cost-effective. Ask the nurseries what they use themselves, and if they'll sell that to you.

My first wintersowing year, I used some Hyponex potting soil, alone and mixed with perlite. Oh, it was bad stuff! However, I still got sprouts, and once the sprouts were out of it, they did fine. Until then, the sprouts were much smaller than sprouts in other containers. Because it can take me all summer to get my sprouts planted out, I want to use a mix that encourages happy, bigger sprouts. But if you don't have a lot of containers and can get your sprouts in the (fluffed up) ground fast, then even junky soil in your WS containers will work (if you have a lot of drainage holes)

Weed-wise: people complain that even the commercial stuff has weed seeds. You can find instructions on sterilizing soil or mix if you want, or just put out one container of your mix that you mark "weeds," and use those sprouts to recognize the weeds as they come up in the other containers. Usually, weeds sprout first :) (But don't yank until you're sure!)

We make incredible compost here, and a great potting mix by lightening it with perlite. But we only use that for potting up, not germination. Enough grass seeds are in the compost to be annoying and to require weeding - not something I find ideal when the intentional seedlings are tiny.

I like ProMix or Sunshine mix for winter sowing and have had no weed problems with them I don't care for MiracleGro. Between weeds, rocks, large wood chunks, and it's tendency to dry out to fast in my growing conditions it's just not welcome at my house. I've used it when I have not been able to get the other stuff and now just plan ahead so that I always have ProMix and Sunshine mix on hand instead.

Lynda

I tried the composted leaves stuff last year when I ran out of soil. It worked okay, but a lot of it was still very large especially in comparison to petunia seeds. Potting mix worked much better.

I wouldn't use anything else, put out about 800 containers every year and have near 100% germination every year as well.

Fran