I would take the tree out of the soil and wash the roots to get rid of the sour soil. Drill holes in the bottom of the barrel. I usually do four about 3/4 inch in diameter. Fill the barrel with a good fast draining mix and work in some slow release citrus fertilizer. Water once a week with enough water to run out the drain holes. Al

Yes, mar50, you DO need to start all over again with some fresh container (non-soil, but peat and bark based) medium. You might want to prune off any dead (brown, black, slimy) roots while you are at it, too.

Oh, and DON'T put anything in the bottom for 'drainage'. All you accomplish is slowing down the process of drainage. Whatever you put in the barrel should be a homogenized mix, no layers of anything. Place your barrel on something to elevate it a teensy bit from the surface on which it is sitting. It won't be able to drain freely, otherwise.