I know you said soil, but is coco out of the question? You can get a brick that expands to 15-20 gallons for about 10 USD and then you can add the guanos and or other organics at your liesure. Coco is nice cuase there is nothing it in, it likes low pH and if you add some perlite it's an airy mix for sure. Also the water retention is great and it still gets O2 to the roots. Another nice thing about Coco is you can make a couple mixes depending on the age of the plant. i.e. just coco and perlite for babies and seeds, then when they are grown up and need a transplant you can add guanos and other organics as you need them. It also flushes nicely.
If you must go soil I like Promix and FF, FF is a bit much but they sell something called light warrior, made for cuttings and seeds. Whatever you decide on just remeber most seeds like no food when they germ. Another reason I like Coco and FF Light Warrior.