I've always though of spirit as the fifth, and void as the sixth. Spirit is what binds it all together, void is what gives everything definition by its absence. Well, that's a rather simplistic set of definitions, but I'm at work, so...

Have you ever looked at some of the Eastern elemental systems? They tend to separate out earth-that-is-alive-and-growing, and earth-that-is-mineral. I like that; it feels right to me.