Just a couple of months ago, we transplanted a young fig start (maybe 3 years old by now? purchased when only 2 years old?) into the ground.
I just *knew* that the poor thing's roots were going around and around the pot, even before we took it out of the pot. So...
DH dug a big hole. Much bigger than you would expect would be needed. We backfilled it partway with some of the excavated soil. (This soil had been, several years ago, improved with our homemade compost.) THEN I took the fig out of its pot, scored down the sides eight times (yes, every 45 degrees), and trimmed an inch or so (maybe 3 cm) straight across off the bottom of the root ball.
We placed the fig into the hole and watered the bottom.
We filled the hole with compost and potting soil and watered again.
We "over-filled" the hole to allow for future settling and watered yet again. (I may have over-corrected at this stage; I now have a "crown" rather than an almost-flat surface.)
So far, the tree has kept its leaves, even though it was in a growth stage when we transplanted it, and has valiantly set a couple of figs!
I also watered in liquid kelp a couple of weeks after we had placed the tree in the ground.
USDA Zone 9/Sunset Zone 17