Earth (tu) is one of the Wu Xing, the five phases of Chinese cosmology, and it holds the central place. It is the phase of stability, nourishment, and balance, associated in the classical correspondences with the centre rather than a compass point, the colour yellow, the transitional period of late summer, the emotion of pensiveness or worry, and the sweet taste. In the generating cycle (sheng) Earth produces Metal, ores forming within it; in the controlling cycle (ke) Earth restrains Water, damming and absorbing it. These cycles of mutual generation and control order the five phases and underlie their use in Chinese medicine, the almanac, feng shui, and fate calculation. These are the conventions of the Chinese cosmological tradition, in which Earth, at the pivot of the scheme, mediates the turning of the other four phases through the seasons.
Earth occupies a distinct structural position among the five phases: rather than sitting between two neighbours on a single ring, it is frequently placed at the centre, mediating the other four, with late summer or the transitional turn of each season sometimes assigned to it instead of a season of its own. In the generating cycle (Sheng) Fire's ash produces Earth, and Earth in turn yields Metal, which is mined from it; in the controlling cycle (Ke), Earth dams and absorbs Water, while Wood controls Earth, as roots break soil. The Huangdi Neijing correlates Earth with the spleen and stomach, the sweet taste, the colour yellow, and the centre, and treats it as the phase of nourishment and transformation of food into substance.
Sources Science and Civilisation in China, Volume 2: History of Scientific ThoughtJoseph Needham with Huangdi Neijing (The Yellow Emperor's Inner Classic)Traditionally attributed to Huangdi; actual authorship unknown