Minecraft wiki entry for Farming, associated text: Mycelium and Grass Block: Mycelium, like grass, can be allowed to spread through plain dirt, then harvested with any Silk Touch tool.
Podzol does not naturally spread through dirt, but growing a giant spruce tree converts all dirt nearby to podzol, which can be harvested with Silk Touch.
: Either form of nylium does not naturally spread, but can be made to spread through netherrack by use of bone meal. It can be used for decoration, or for growing Fungus and other Nether plants.