erm .. you got some seeds you just sprouted but no soil ?
in general it don't matter that mouch .. as long as you get a soil that is with in the PH ranges weed like ..
witch is between 6 and 7 .. a soil with a PH on 6,2 to 6,8 would be optimal ..
don't go cheap here .. it can give you problems all grow ..
weed need a soil with the right PH in order to take up important nutrients for its grow ..
beside the right PH you should try to get a airy soil as weed like a soil with good drain ..
a soil witch keep 30% perlit or leganuts is perfect .. coco fibers can also be used ..
now if you have a soil mix with a PH of 6-7 with 30% perlit you have a perfect grow medium for weed ..
now its only up to you if you want it organic or not ..
and if you want one that is for seedlings (= low on nutrients in the soil = you need to begin to add nutrients early on .. maybe week 2) or a "normal" one that maybe keep enough nutrients (NPK) for the fist +3 weeks
p.s
good things to add to a soil mix
Dolomit Lime (make sure to keep a stedy PH)
Epsom salt (magnesium sulfat witch make the plant take up nutrients easily)
Kelp meal (have a lot of trace elements and is good for early grow)
and the list actually go on:
http://www.growshopalien.com/onlineshop/contents/en-uk/d28_organicsandadditives.html
but try to keep it simple here in the start .. don't get confused just try to get a good basic soil with the right PH and Perlit in it .. then you cant go totally wrong and then some basic nutrients made for soil/terra ..
you can do more research and experiment more on your next run .. may I suggest you then buy the soil in time .. also if you want to mix your own organic soil at some point .. as its a good thing to mix it 3-4 weeks before use if you want to add some nutrients and want a nice micro life in the soil ..