Also known as: Yang Earth · Mountain Earth
Wu (戊, Wù) is the fifth of the ten Heavenly Stems (天干) used in Bazi and the Chinese calendar, and represents yang earth.
Wu is yang earth, classically the great mountain (城牆之土). It is the firm wall, the embankment that contains rivers, the bedrock under the city. Wu Day Masters present as steady, conservative, trustworthy, slow to anger and slow to change — but once their position is set, very hard to move. They make natural protectors, guardians of family, and stewards of long-term institutions.
Wu pairs with 癸 in 戊癸合化火, where great mountain meets gentle dew and ignites distant fire — often glossed as the chart of late-blooming insight. Wu fears too much wood (forest cracking the rock) and welcomes water in moderation (rivers carving valleys productive).
Wu holds its salary position at Si (巳), its blade at Wu (午), and its growth stage of birth at Yin (寅), the same stage as Bing fire, following the old doctrine that earth is generated alongside fire. It roots in Chen, Xu, Chou, Wei, Si, and Wu. Judging a Wu day master means checking whether the month is an earth or fire month, counting earth roots below, then looking for Jia wood to loosen the soil and Ren or Gui to moisten it. Reverse that order and the commonest misjudgment follows, that lots of earth means a strong day master. Three Wu in the stems over Hai, Mao, and Wei is wood-strong and earth-thin, a weak chart; a single Wu over Chen, Xu, Chou, and Wei is genuinely thick earth. Seasonally, spring Wu wants warmth from Bing and is wary of overgrown wood, summer Wu cracks dry and wants water, autumn Wu is drained by metal and wants fire, and winter Wu freezes without fire.
Di Tian Sui describes Wu as solid and weighty, centered and upright, and characterizes its function as closing when still and opening when in motion, hence the claim that it governs the course of things. It then adds that moisture lets earth produce while dryness makes it sick, which is the source of the later rule that earth should be damp but never saturated; the preference for water in moderation noted above is the same measure stated differently. The closing line, about positions at Gen and Kun preferring stillness to clash, concerns the mutual clashes among the earth branches Chen, Xu, Chou, and Wei.
Three disputes cluster around Wu. The first concerns clashes among the earth branches: one line reads Chen clashing Xu as opening a storehouse and releasing the hidden stems for use, so that if those are the stems the chart wants the clash is welcome; another holds that the clash shakes the roots and thins the earth outright, leaving the day master less supported. Both readings remain current, and in practice what matters is whether what emerges is wanted. The second is the rationale for placing Wu's stage of birth at Yin alongside Bing, since the idea that earth shares fire's origin has long been questioned. The third is when earth is in season: the closing eighteen days of each season, or a separate late-summer rule.
Wu (戊 wù) shares a root with the word for luxuriant growth, and Han glosses on the stem names read it as the stage at which plants reach their fullest density. That is an older layer than the mountain-and-rampart metaphor, and closer to what the stems originally were, marks along a cycle of time. The placement of Wu and Ji at the center, with earth as their phase, comes from the Liji “Monthly Ordinances” and the astronomical chapter of the Huainanzi, and that is the textual basis for assigning Wu to earth at all. The Jing Fang pairings run differently again, assigning Wu to Kan, the water trigram. One note in passing: when the classics call Wu solid and weighty they are naming a mode of action, bearing and defending, and not a body type or a reaction speed.
Wu and Ji are told apart by function, and quantity has nothing to do with it. Wu is high ground and embankment: it blocks and bears weight. Ji is field soil: it holds and nourishes. Their requirements often run opposite, since Wu is wary of water that breaches the bank while Ji wants water to keep the field workable. Among roots, Chen conceals Wu together with Yi and Gui, while Xu conceals Wu with Xin and Ding, so the two behave differently in use. Gui is both Direct Wealth and the bonded partner of Wu, a double relationship easy to overlook; and the transformation of that bond into fire has its own requirements, normally a fire month with the transforming phase well supported and no competing stem, so the bond alone decides nothing.
A chart whose day stem is Wu (戊 wù) is a yang earth chart, pictured traditionally as high ground, mountain, or embankment, and associated with bearing weight, holding a position, and resisting pressure. That describes how the tradition reads the stem; it makes no claim about a person. The working reading depends on the birth month, on whether Chen, Xu, Chou, Wei, Si, or Wu appear below as roots, and on whether wood is present to loosen the earth and water to moisten it.
The phrasing comes from the Di Tian Sui line that moisture lets earth produce while dryness makes it sick. Thick earth with no wood compacts and grows nothing; with no water it cracks. So the traditional first questions about a Wu day master are whether Jia wood appears to break the soil and whether Ren or Gui appears to wet it. Both have limits, though: overgrown wood controls the day master, which is what the caution above about too much wood means, and excessive water breaches the bank. Where the limits fall depends on how thick the earth already is.
Not necessarily. The traditional order of judgment is season first, roots in the branches second, and the count of stems last. Three Wu in the stems standing over Hai, Mao, and Wei face strong wood controlling thin earth, which reads as weak rather than strong. Conversely, a single Wu over Chen, Xu, Chou, and Wei sits on very thick earth. Counting stems without looking at what supports them below is the easiest way to get this wrong.
The tradition separates combining from transforming. Wu and Gui combining establishes a relationship; the pair actually becoming fire is a further claim that normally requires the month branch to be fire, the transforming phase to be in season and well supported, and no other stem breaking the bond or competing for it. Where those fail, the standard reading is combined but not transformed: the two stems restrict each other without changing phase. Schools differ on how strict to be, which is worth stating when the reading is given.