I start out with Schultz's seed starting mix that i get from Walmart, but i never remember to get more and they run out of it early. After that i use the ProMix i buy in fall for containers. Possibly, you could add vermiculite or peat moss to your black earth and that would lighten it up a bit.

I will be using ProMix this winter, but any other Potting MIX should be O.K. Just check out the ingredients. I would think any SOIL would be too heavy. MOST mixes do not contain any nutrients and I apply a weak solution of fertilizer to the seedlings to help them along.

I also use Schultz's seed starting mix that I get from Walmart. Works good for me.

I am using pro-mix for professionals that I buy from a greenhouse or a good garden center. To sow regular perennials or annuals, I use only pro-mix but when I sow more delicate plants such as alpine and rock garden plants, I use pro-mix and add grit or small vermiculite, half and half. The mixture allows a perfect drainage and it is easier when comes the time to divide the seedlings and transplant. Sometimes, especially for sowing paeonies, I use only pure sand. Using soil is dangerous as the seedlings will have more tendancy to damp off.
If you need more help, you can contact me.
Denyse,
President of the Quebec Alpine and rock garden Society

Here is a link that might be useful: Quebec Alpine and Rock Garden Society

Pro-mix, the largest Professional bag or a similar seeding mixture from Home Depot which has less vermiculite is a soil-less mix I've used for years. I also add grit to rock garden seeds. It has been well worth the extra dollars. Less trips to the store. I reuse the soil-less mix in hanging pots and large planters or plant 'as is' in gardens.

Compressed bails of either Sunshine #4 or ProMix.