Rather than consider fertilizer, I'd take a long look at the consistency and quality of the potting medium you're using...AND the watering frequency.

I don't know where you live, but in most temperate locations, you should be dramatically reducing the frequency of the irrigation cycles as fall approaches. It's not good for in ground OR containerized plants to be moist all of the time. Those grown in containers are less forgiving.

thanks

the zone is new york-south, the location gets minimal sun, and yes the soil always seems moist as it is on a drip irrigation timer. the same "always moist" conditions are true for three other trees we have in large planters. the soil used was formulated potting mix from a bag. do you suspect that "always moist" soil could be problematic? i debate this point with my wife all the time especially over this hot/dry new york summer.