If you haven't tried the NuMix from growersolution.com, you might want to read this.
For less than $11 per bag, 2.7 cu., you get enough 'dirt' for 11 trays of 1203 liners.
this 'dirt' is comparable to MiracleGrow. It's made in TN from pine bark, worm casings and a starter fertilizer. I don't know what kind.
I picked mine up when I was visiting the area. I'm impressed so far, just opened and used the first of 4 bags. I purchased the NuMix2 which is finer than the NuMix3. I had very little stems and I found no big chunks or bigger twigs. All of dirt would fit into the 1203 cells with no problems, it probably would have been able to be used in 128 or 150 cells, but I didn't try them.