Fill in the blanks with corresponding chemical terms: (1) It exists in a small amount in human gastric juice to help digestion: _ _ _ _ _ _. (2) It can be used as compound fertilizer in agriculture.

(1) gastric juice contains hydrochloric acid, which is helpful for digestion, so fill in: HCl；;

(2) Potassium nitrate contains potassium and nitrogen, which belongs to compound fertilizer, is a compound composed of potassium ions and nitrate ions, and belongs to salt, so fill in: KNO 3；;

(3) The most abundant metal element in the earth's crust is aluminum, so fill in: al.