I'm not sure when you're supposed to unlock artificial soil (I think with landfill (?)), but it's never unlocked. I've also tried creating a new game in sandbox mode with all the achievements unlocked, and still, I can't craft it

Lol whoops, fixed

Thanks, that was quick! Awesome mod btw
I have another suggestion (I don't think it's a bug): have you considered allowing the artifical grass to be placed on landfill?

I have, and intentionally did not for balance reasons, but I may make it a setting. It does feel like you should be able to, but since artificial grass consumes pollution (as do the trees plantable on it), it can be abused to suck pollution out of the air.