If farmland exists within four blocks of a Water Source Block, it will become hydrated and will grow crops faster. If farmland is left unhydrated and unplanted, it will "dry up" and revert to dirt over time. Farmland can also be found in villages with wheat, potatoes and carrots growing on it.
Dry Farmland (left) and hydrated Farmland (right).
The max amount of Farmland that can be hydrated with one block of water.