Podzol

Podzol is a grass-type variant block that will be featured in the Redwood Forest in 1.7. Similarly to grass blocks, it can only be obtained by using the command, the creative inventory, or by digging it with a Silk Touch-enchanted tool; otherwise, it drops dirt instead.

Podzol will allow mushrooms to be placed on it no matter the light level, and allows the growing of giant mushrooms, which suggests it's similar in coding to Mycelium

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Trivia

 * When Podzol has Grass, Dirt or Other Podzol blocks placed on top of it, its texture changes to the dirt texture, but does not actually change to dirt. (Mycelium does not have this effect.)
 * Podzol does not spread to adjacent dirt, though this may be a bug.
 * Podzol appears to be comprised mainly of small grass scraps, dead leaves and moss.
 * Grass cannot grow on Podzol
 * In real life, Podzol is the type of soil usually found in coniferous forests in wet, cold areas as well as warmer areas.