The falling leaves look perfectly healthy. I have tried keeping the soil damp and that hasn't helped. I just purchased this tree about two weeks ago.
My Lemon Tree Is Losing Leaves
Certified GKH Gardening Expert
I suspect that this is either a watering or nutrient issue (or combination of both). It is my recommendation to review the watering and fertilizing guidelines for citrus trees in the articles below:
Fertilizing guidelines for lemon trees:
Tips on watering your lemon tree:
Causes of leaf drop in lemon trees: