I think keeping plant-based spells under Wood (a living thing that can be manipulated after it's dead) and metal-based spells under Metal (a mineral separate from Earth that is never alive but can flow like a liquid if heated, unlike earth which mixes readily with water, whereas metal doesn't fully mix with water, or something something etc.) is best ^^;
So, what about flesh-based spells like transformation/size change/healing/blood/bone/organs like eyes and ears, etc.? Blood can be the rare mix of Water and Metal (iron), but a simple Enlarge/growth spell would involve flesh, bone, blood, organs, the whole gamut working in concert to increase a creature's size (may also involve Spirit)
Also, how would Light and Darkness be classified? I can imagine Light being a subset of Fire (even cold chemical light like fireflies), and perhaps Darkness too (absence of light, plus ashes/soot being a product of Fire)? I dunno...
Finally, electricity (lightning bolt, chain lightning, St. Elmo's Fire, etc.) could be a subset of Fire I suppose, although the way it travels through Metal is unique (fire could use metal as a conductor, as in heating one end of a piece of metal, but it's not as efficient as electricity). Also, electricity can melt metal if hot enough, so definitely related to Fire there. Yet, electricity not only burns flesh, but causes paralysis by shorting the nervous system. An active nervous system is a required in most forms of life, therefore one could extrapolate that life is a subset of Fire, therefore Raise Dead/Resurrection type of thing might be related to "restoring the fire of life"?