User blog comment:IamJakuhoRaikoben/Chapter 379: Tartarus Arc, Part 3: Underworld King, Review./@comment-49.184.77.110-20140404122626/@comment-7418318-20140404123918

Makarov did say something like "When the Spirit flow within us combines with the Spirit flow in nature, magic is born". Maybe that has something to do with how Franmalth is able to absorb both Souls and Magic.

It would make more sense to me than individual Spells having a Soul.