This is a request. I hope someone will code this mod
1) Oil. If oil spill on dirt - 3 cubes from the grass to the dirt make the dirt unsuitable for plant's growth. It's possible to reestablish the dirt via some substance;
2) Lava. If lava spill on the dirt/grass - 1 cube of dirt/grass makes unsuitable for plant growth. it's impossible to reestablish the dirt;
3) Biomass. Generates in world like remains of animals and plants.
And so on.
Also it would be nice to add slowly moving while walking on the contaminated dirt.