The object of the invention is to provide a bioactive agent in which the bioactive effectiveness of ferric iron salt and/or ferric ferrous iron salt contained therein is stabilized, making long term preservation possible, so that the bioactive agent can be useful as an original solution for medical use, as a soil conditioner, or the like. An iron-magnesium mixture solution is provided in the invention, wherein the iron-magnesium mixture solution is produced by mixing an aqueous solution containing a ferric iron salt and/or a ferric ferrous iron salt in a concentration of not less than 0.5 mol/L as iron in the ferric iron salt and/or the ferric ferrous iron salt, and an aqueous solution containing a magnesium salt in a concentration of not less than 0.2 mol/L as magnesium in the magnesium salt.
