I think that the bag of soil should not be made from wood, or at least, not strictly so. Anything which decomposes should be possible to make into soil, or just like... dig it up. At any rate, I am saying this because I did not end up using this at all for my games because wood was too precious on grassland biomes.

I originally had soil made of hay, but changed it to wood to be simpler as I didn't want to make the requirements too complex.

This is the same reason why compost used to be made using a mix of food products and hay. The functionality worked, but it was too much work for the player to gather a certain amount of nutrients in food and hay.

Any suggestions on what to change it to? I could maybe add an option in the mod settings to use an alternative material for soil.

I feel like Rich Soil (with full fertility bonus) should be possible. It can be difficult and expensive, maybe hidden behind a lot of research or something. But it shouldn't be completely outside the realm of what is physically possible. I really like the mod other than that. Would it be possible to add this? Maybe make it a mod option? Thanks for the most balanced terraforming mod out there!

I did this because having the ability to make fully fertile soil runs the balance of the game a bit, removing the challenge of the randomly generated maps. It would definitely be possible to add this, but I haven't worked on the mod or played Rimworld in a while now. Whenever I pick it up again, I can definitely look into adding it as a toggleable option that's disabled by default. In the meantime you can spawn any terrain tile by using Dev Mode, which you can enable in the options. There's a tool called "Set terrain (rect.)" or something.