| | Re: dirt
Not something I would worry about.
Your substrate will constantly be replenishing its nutrient content.
Since it's your first time using dirt, I would advise you experiment on something a little smaller than a 100gal, just to get a feel for working with the soil, and how the tank will mature and develop. Even just a 1gal jar.
Also, read Walstad's book if you haven't. I firmly believe you're shooting yourself in the foot by not doing so.