Can Control( Philosophiae ) control elements or just animals?
The Elementalist Technique Controlling(Philosophiae) says it controls natural things. So a Elementalist - air with the controlling(Philosophiae) can control animals with the Phlegmatic trait. But there is no mention on controlling the natural element or any guidelines for it although it would suggest that it could.
I could assume that the level/amount of control is based on the summoning table since it is used as an example of Philosophiae summoning but it would be a guess.
I would say you can. As it says "An elemental philosopher can affect the motionof natural materials with the appropriate element, or organic materials using his lowest Form as a requisite". It then goes on to talk about using this to help your soak.
I think it would allow you to replicate Rego (form) effects of up to a slightly unnatural level. So say you wanted to move a bonfire to attack someone - it's size is in the 0 to +3 region, so I'd say a level 20 on stress die + presence + controlling + form would allow you to do that. Now, how unnatural do you let this get? Can a fire leap many paces at someone, or you force a very distant storm cloud to move over here and rain and thunder at someone? I think that's a matter for negotiation with your SG, but as it costs a fatigue each time an Elementalist does it I don't think it can get too game-breaking. (Yes, theoretically someone could make a city move with control + lowest form of 45, but that's an art + difficult art total and Elementalist longevity rituals don't allow very many to get that far)
It's explicitly stated the (Philosophical) Elementalism can affect raw elemental material as well as animals associated with the respective humours, but unfortunately no specific guidelines were included in HMRE.