The current state of substrate for planted tanks

You could do a mineralized topsoil capped by fine gravel or pool filter sand—grassy things like hairgrass do generally favor a fine, nutrient-rich substrate. 1.5" of dirt capped by 1.5–2" of sand should be sufficient.

Aquarium "soils" (Aquasoil, Florabase, Fluval stratum, etc.) would also work, but they carry a much heftier price tag. Depends on how much you're willing to spend, I guess.

I found this sites with a substrate calculator:
http://www.aqua-calc.com/calculate/gravel-rectangular-solid
http://http://www.plantedtank.net/substratecalculator.html

They seem to agree that for 3" average depth (2" in front and 4" back) you would need 90lbs of Flourite, or 144lbs of Eco Complete.

Aquasoil suppliers recomend four bags of 9liters each for tanks close to that size, which if my calculations are right would give about 2.5" average depth. It is about 14 liters of substrate per inch of average substrate depth in a 75 gallon tank.

I've used Turface as a substrate, but that is even less dense than aquasoil and thus the turface can give you problems with getting plants to stay down. Good luck tracking down a supplier though.

You could keep your current mix and replace the front section with the smaller granule aquasoil, unless you were going for the whole tank in "grass".

Oh I forgot I had used Soilmaster Select charcoal for one tank, weightier than Turface, but I had some problems with surface scum buildup with the bag I had. It grew plants decently with fertilization. It didn't grow cryptocoryns as readily as Flourite.