Yes this is may be spoiler for you but i don’t care :D,

on the last minutes of episode 20 book II uncle Iroh trapped by crystals just like King Bumi did to Katara and Sokka, before uncle Iroh trapped he seem doing something like earth-bending move, so… uncle Iroh also mastering earth-bending or there’s another earth nation army??

note : all earth nation army has been knockdowned by Katara