Mud Block

Dirt cannot hold jungle grass or mushroom grass. Mud can. That single property is why anyone carries a stack of it: every artificial jungle, every glowing mushroom farm and every truffle house is built on mud and would not work on anything else. Corruption and Crimson will also creep across it, so a mud farm anywhere near infected ground needs a gap dug around it or it gets eaten.

Chlorophyte spreading through the cavern converts dirt near it into mud on its own, so a jungle you have left alone quietly manufactures more of the stuff while you are elsewhere.