My 5-year-old Meyer lemon is having lots of leaves turn yellow and drop. It's in a 14 inch pot filled with cactus soil and gets watered about once a week, when the surface is dry to about fingertip depth. I've had nitrogen and magnesium deficiencies in the past, but I can't figure this one out. Two months ago it responded pretty well to generous fertilizer, but now both new and old leaves are yellowing.
