Sorbus with curled leaves
I have a Sorbus americana (American Mountain-ash) that is in its second summer growing in a full afternoon sun area in my backyard. It's in clay with a 2" layer of bark mulch on top and gets a heavy watering every other week. The leaves have been curled for the past month or so. As the photo shows, some of them have yellowing along the edge. Despite this, new growth has appeared over the past week.
Are the curled and slightly yellowing leaves a sign of a nutrition deficiency? Because I have manzanitas and penstemons growing in this area, I do not amend the soil. (If so, I may have to dig this tree up and grow it in a large container.) I've given this tree a little extra water over the past couple of weeks, but the leaves remain curled.
Any thoughts would be appreciated.