Those are good questions to ask and I admit it would be quite important to know, so excuse me for not replying earlier. As to the nature of the powers, they are not conjured from nothing. They are created using the elements around you. For example, if the fire-user needed a flame and fire was not in his immediate possession, he could create a spark from hitting rocks together and then expand that spark into actual flame.
As to the other powers...Earth is indeed inorganic and Nature is organic. Earth has all power over rock, but of course their are certain limitations. You may change the formation of the land, but beware of where you do it. For example, if you wanted to make a hill or something of that nature on Fridera, chances are good that magma will arise between the cracks that it causes. Also, try not to use it in great fashion. You know...like causing earthquakes and so forth.
Nature on the other hand, deals with both flora and fauna. You can control plants and animals to certain extents, but nothing extreme. For example, you can have nearby plants entangle foes to incapcitate them for a short time and you can even control other creatures to help you for a time. However, you can't say...cause trees to uproot and attack people or completely control a creature forever.
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There are many things that one can do with their powers, some are fine while others are overdone. If you have an idea for a power that you may use and aren't sure if it's appropriate or not, please refer to me and I'll tell you if it's alright.