just get dirt from outside where no pesticides where used, its free and i wouldn't use miracle grow its going to have alot of fertilizers in it. Then just buy 1 or 2 bags of sand at lowes/home depot to mix with and while your their pick up some 45 outdoor flood bulbs
Make sure the 125 gallon tank has a solid top and you should start building its final cage ASAP. He will be 3ft in no time then by the end of the year hes going to be huge.
good luck
Bryan