It's 5 parts pine bark, 1 part perlite and 1 part potting soil.

I put the mix together in a separate container by measuring out the parts and then stirring it together by hand. When I had it ready I carefully pulled the tree out of its pot. My daughter held it steady in the shade with the root ball in a container of water while I emptied the old dirt out of the pot and replaced it with the 5-1-1 mix. Then I gently rinsed most of the old dirt away from the roots, and I cut off a few small roots that were looking like they might have started rotting. I did this while keeping the roots under water the whole time and in the shade. Most of the roots looked healthy. Then I put the tree into the pot and poured more of the mix on top to cover the roots. After that I thoroughly watered it. The tree was out of the pot for probably 5-7 minutes.

I hope I did it right! I'm not supposed to fertilize for a bit now, correct? I think the tree will be happier now. I was surprised how much water the old soil had retained. Near the bottom it was almost sopping wet.