Earth is the pivot of the Wu Xing, and its handling differs enough from the other four that the difference is a feature of the system rather than an untidiness in it.
In the generating cycle, sheng, Fire produces Earth as ash, and Earth produces Metal as ore. In the overcoming cycle, ke, Wood restrains Earth, roots breaking it up, and Earth restrains Water, banks and dams holding it. Those four relations define the phase.
What sets Earth apart is its position in space and time. The other four take a cardinal direction and a season. Earth takes the centre, and its temporal assignment is disputed within the tradition itself: one scheme gives it late summer as a fifth season, another distributes it across the last eighteen days of each of the four seasons, so that Earth is present at every transition rather than occupying one of its own. Both schemes are old and both are still in use, and a source that presents either as the only arrangement is simplifying.
In the classical Chinese medical literature Earth is the phase of the spleen and the stomach, of the mouth, of the flesh, of the colour yellow, of the sweet flavour and of pensiveness among the emotions. Those are correspondences recorded within that system and are set down here as description only.
Yellow, the centre and the earth together carried imperial weight, which is one reason the phase attracted political argument as much as medical.
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