Does anyone know the reason why 生地黃 is called 地黃 [yellow earth]? I don't know if 六味地黄丸 took the name from 生地黃 or viceversa, but either way I couldn't find neither the etymology of 六味地黄丸 in English (I still can't understand Chinese properly). Any idea for the reason of the presence of 地黄 in the two?
黃 is sometimes a plant name: 大黃 'rhubarb' (this used to be dai-huang, not da-huang), 麻黃 ephedra'; 地 sometimes means a tuber, seems that's not the case here. One source identifies 3 varieties: 天黃, 人黃, 地黃. This is the 三合 system.
地黄 is the plant, Rehmannia glutinosa. For TCM, it can be used in various ways:
生 (raw) is usually a synonym to 鲜 (fresh), but here also means unprocessed and therefore equivalent to either 鲜 (fresh) or 干 (dry).