The skills look good. though there are a few things with the passives:
Qpassive: too weak. 2% damage increase means nothing, and 6% doesn't either. you get up to 12 extra base damage on rank 5! which is would you would get with 20 AP too. make it 3/6/9/12/15% for it to be noticable (he has to get in normal attack range to get this damage anyway). Also, do burning enemies take DoT? if they do it could mean lich bane would be a good item on fire-focused Sorotos.
Wpassive: too strong. you passively get more health than a 1110 gold giant belt, and armor as a bonus. the armor itself would be stong enough already, since on casters it would increase their physical resistance by about 10% late game, and even more early game due to the low initial values.
Wactive: another a close range skill kinda ruins the thing mages excel at. it would be better to grow an earth elemental at target location that will be the epicenter of the earthquake.
E: I like the passive, promotes coming closer even more than auto attacking with Q. But I don't think that the active synergizes well with the passive. if the passive means they circle around him (i'm assuming at a speed of something close to mundo's W but a bit slower) how are you going to get people to be affected by the passive? unless you increase the range of course..
I had an idea for a more fun active. what if you fire one of the air orbs in a straight line towards your cursor (like irelia's R), pushing back everything it touches by around 100 range? you can use the passive to let them come close and let them suffer for it, or use it not to allow them to get close with pushing back.
R: I don't think an ultimate fits a champion that uses all 'elements' equally. Like udyr I think you would be better off having no ultimate, and just another skill, with just another passive. the mobility increase could fit water very well.
passive: I don't think the spirit realm fits in line with the entire elements thing, as well as the passive movementspeed increase being too powerful (miss fortune has up to 75 movementspeed from her passive, but that's without being attacked, and builds up over time).
I think there is potential here for something really powerful though. Do you know the game Magicka?
something similar could fit here, combining elements for additional effects.
fire = burning enemies, earth = the earth elemental you cast, air = the orbs orbiting you, water = enemies made wet by the water skill.
when you use another element on an enemy, new effects are applied:
fire+earth / earth+fire: lava
the burning/earth elemental turns into lava over the entire area of the skill, burning and dealing additional damage. o(note that the area of the burning is only the affected champion)
fire+ water/water+fire: steam
the water/fire on the enemy is turned into rapidly expanding steam, dealing damage and pushing everything around the affected enemies away 150 range.
the powerful wind makes the flames rage even more furiously, the air orbs set fire to everything they touch, and the already burning enemies take 25% more damage from burning and have the burning debuff refreshed.
it may be complicated, but awesome. even if you do not wish to include the passive, a more udyr-caster type of character would fit him a lot better. all skills should have 5 ranks, allowing for specialization in some elements because you can't get enough skillpoints.