"Salt" could be an earth spell. Could cancel out ice attacks/elementals, and could make water sheilds vulnerable to electrical attacks.
In fact, a sufficiently powerful earth spell could increase the amount of metal in the whole battlefield (or at least beneath the enemy).
For instance, if you made the ground beneath the enemies lined with copper or diamond, then any electrical attacks in the area would attack ANYONE on that altered ground, and more effectively (max damage, central damage at all points).
Especially since Electric attacks, one possibility at least, is more damage for less area of effect.
Using high level earth spells to comliment your air spell could make it much more powerful.
Of course, a proper Earth Spell (without conductive Materials), would completely nullify any lightning based spells indefinitely, but would be incredibly weak to earth based spells, unless it was a Stone-shield.
Meanwhile, Wind, Fire, and Water work on a more percentage basis than Earth and Electricity.
For instance, Equal amounts of Water and Fire cancel each other out, and equal amounts of Ice and Magma cancel each other out. However, it takes only half as much Ice to cancel out fire, and only half as much magma to cancel out water.
It takes twice as much wind to cancel out Fire, and 4x Wind to cancel out an equivalent Earth spell (blow the earth out of the way). In addition, Earth spells of OIL can increase the Area of Effect and Damage of Fire spells extensively. Air spells of Oxygen can increase Firespells even more. Using Oil and Oxygen spells can stack, to truly make devastating Fire spells tripled in strength and Radius.
So Wind can block against Fire and Water, albeit more expensively. Water and Fire can cancel each other out in equal amounts, and any amount of Earth Shield will shield against 10x as much electirc damage, and any Stone Shield 10x as much wind damage. However Wind damage is twice as effective against an Earth shield, and Electric Damage is twice as effective against soldiers standing on conductive metals (at least twice). I suppose Stone Shields are effective against electric attacks as well??? only Stone Shields are twice as expensive, for being effective vs Electric AND Wind attacks. Although, I would find it more interesting if Stone Shields would inadvertently have just enough conductive Materials to cancel out the effectiveness against electrical attacks, and thus it is merely a shield against Wind. Therefore Stone is neutral to electricity and effective against wind, and "Dirt" is weak against wind but effective against Electricity (For sake of argument).
Stone and Dirt could be equally expensive, and then doubly expensive would be "Lead Wall" which is effective against both. Dirt would be able to defend against Twice as much Fire-magic. While stone and lead would be able to defend against 5 times as much Fire magic. Magma, however, is nuetral to Dirt, and only half as effective against Stone and Lead. (as opposed to Fire being one fith as effective).
Water and Ice would be Neutral to Electric attacks (no protection), while Water and Ice give "some" protection against Wind attacks, yet not as much as Stone. Fire is weak against Wind, yet weaker to Water and Ice.
Dirt offers almost Total protection against electricity, however weak to wind. Stone offers no protection against electricity (given the second assumption) and some protection against water and ice, yet much more protection against Wind. (optionally you could have stone somewhat effective against electricity).
Stone and Dirt require some protection against Fire, stone moreso. Magma is in some ways like a "stronger fire" yet Dirt offers no protection against Magma, and Stone offers some protection against magma.
Dirt and stone are weak to water and ice, however Dirt is weaker than Stone.
Lead offers protection against everything (other than arcane) although it offers the most protection against electricity and Wind.
Fire and Water offer Massive Protection against each other, and any excess in one or the other gives a Logrithmic Advantage. (for instance, a little extra water gives some water damage to the person protected by Fire, however a medium amoung ot extra water gives extreme water damage to the person protected by Fire)
If I missed anything (I know I repeated myself alot) let me know. Essentially im saying that we could be more scientific, and hopefully it wouldn't require damage types, simply "generic rules". While much might not be implemented, I would like the Conductive metals Idea and Salts idea.