Tried out the Fertilized Dirt, which was interesting but only seems to work properly with 3 vanilla crops.

Beetroots pop off as soon as you plant 2 next to each other, or the first time they get a green sparkle "fertilizer" tick. So they always pop off before they can grow.

Melon & Pumpkin stems will plant on the Dirt (once it's tilled) but work grow much more slowly, might be unaffected by the growth acceleration.

Haven't tested Sugarcane or Tree saplings.
Edit - Further testing on Melon/Pumpkin stems, updated info.

Correction - Pumpkin & Melon stems grow, but very slowly as if they are unaffected by growth boosters on the Fert. Dirt (both the Dirt & Extra Util's Water can were used with no effect for quite some time, emptied and refilled the can multiple times).

This may be related to (or the same issue as) #114