I have a Fukushu kumquat growing in a 10 gallon air root pruning container. I noticed a few small branches dying back. I thought it was not getting enough water, so I increased the amount and frequency of irrigation. The die back continued. Therefore, I thought it probably needed re-potting. When I pulled the tree from the container I quickly seen that the water was channeling through the medium. A good part of the medium remained dry even though I applied a good quantity if water. I removed a good portion (but not all) of he old medium and replaced with a 5-1-1 medium (5 parts Repti-Bark, 1 pert peat and 1 part popcorn sized perlite and soaked the tree for 4 hours. Be careful with large containers and watch for signs of trouble, and take action quickly when signs warrant it.