You do not control the forces of nature, you don't even command it; you do something greater, you ask it.  Sometimes it replies, sometimes it doesn't, and sometimes it becomes greater than any magic out there; for you know it's better to work with life than to control it.

Any time the player casts a nature spell, they roll a D4 to see if it will work.  Results being:

1-Nothing happens
2-Normal roll
3-Crit roll
4- Nature roll (DM makes rules, and rolls for nature)