The title element of Book Two of Avatar: The Last Airbender is Earth. The second season follows Aang and his friends as they travel through the Earth Kingdom while Aang searches for an earthbending teacher.
Book Two: Earth aired in the United States from March 17 to December 1, 2006, and contains 20 episodes. Its major new character is Toph Beifong, a powerful blind earthbender who becomes Aang’s teacher. The season also expands the story beyond the group’s journey by exploring Ba Sing Se, the Earth Kingdom’s capital, and the political control maintained there.
The title fits both the season’s setting and Aang’s training. Earthbending requires him to confront obstacles directly and develop a style very different from his natural airbending instincts. The three book titles correspond to the elements Aang must master after air: Water in Book One, Earth in Book Two, and Fire in Book Three. Air is therefore the first book’s element, not the second’s.