i personally watch those cotyledons like jartlow stated above when they start to show some yellow starting with a very small dose then would be an ok time as far as soil goes i grew up on a dairy farm and use a mix of haylige compost (alphalpha, hey, and corn compost) and this spot where we have buries dead cows for years and the soil is just really nice and rich
before using this i would go with my local grow stores own mix as it was cheaper ad\nd always did worked well