You can use peat moss. A base substrate for seeding and enriched substrate for transplantation (or a mixture of both at a ratio of 1:1). In any case you have to be careful with substrate pH which must be about 6.

Regards

it depends to seed kind, for example i use usual soil, to produce Iranian satureja spp.

but sage can not be germinate in this substrate. Also sometimes pH of substrate is very imporant (cress). and ..... 

Seeds of several species of native salvia spp Iran. The seeds sown in trays that containing  coco peat + perlite(1:1) medium but contamination whit the fungal diseases. then seedling transported to  pots but contamination whit the fungal diseases. we used fungicide before seed culture. we applied  medium container ratio of 1: 1: 1 soil + sand + cocopeat in pot.

The soil pH is 9 .

how can reduce to 6?

You can mix it with peat moss having low pH.

I like to sterilize seeds first with 1% NaOCl and sow them on agar. Then they could be planted in according to the need of the plant and the pH.