己 Ji
Yin Earth, garden soil, the cultivated field. Earth that has been worked: fertile, receptive, and quietly making things grow.
What a Ji day master is
Yin Earth is not the mountain but the topsoil. It takes in, nourishes, and produces. Ji people are usually the reason something else flourished, and they are frequently uncredited for it because their contribution looks like conditions rather than action.
In Chinese astrology your core element is Yin Earth, pictured as garden soil. You are nourishing and humble, the quiet reason things around you grow. Everything you tend gets stronger.
Ji at its best, and where it strains
Strength. The ability to make other people and other projects productive. Ji improves whatever it holds.
Difficulty. Soil gives itself away. Ji can be depleted by continuous giving and can absorb other people's problems as though they were nutrients rather than weight.
Ji shares its element with Wu, the yang form of Earth, and the two behave nothing alike. That is the whole reason BaZi uses ten stems rather than five.
The four relationships every BaZi reading runs on
Your day master is not read alone. Everything else in the chart is judged by how it treats Earth, and the five elements move in two cycles: one that nourishes and one that restrains. For a Earth day master those come out as follows.
A chart with plenty of Fire and little Wood reads very differently from the reverse, even with the same day master. That balance is what a BaZi practitioner is actually looking at, and it is why two Ji people can be unrecognisable to each other.
BaZi pillars here use the solar calendar the tradition uses, so the year turns at Lichun in early February rather than at the lunar new year, and the hour pillar follows the late-Zi convention.