It is normal for citrus to drop older leaves, especially on established trees like this one. And it's always a good idea to fertilize regularly. But on the third leaf from the right, I think I see Whitefly larvae and their white stuff they leave around them. And I think the first two leaves may have a little bit of sooty mold, which would come from the Whitefly larvae droppings, a sweet sticky substance. It looks like a pretty mild case. On a tree that size they won't really damage it, it's more of a nuisance then. But if I were you I'd take the hose with a spray end and give it a good hard wash with water, especially on the under sides of the leaves. That will get rid of a large percent of them, then I'd do that every week for a while, say a month or so. That will help get rid of new ones as eggs left behind, hatch out. Overall though, johnmerr and pip are right, it's in pretty good shape, just need some food.

Looks like your trees might be suffering from some bark disease. You need to remove the landscape rock from near the trunk out a couple feet, it could be trapping unneeded moisture too close to the trunk and be causing that bark splitting. Can you take a close up of the trunk on both trees and post the photos? The one leafless branch on the upper photo looks like it's had twig die back, trim it back to where the branch looks green which could be at the node where it grew from. What is your grow zone? How often do you water and fertilize? Evan

This is most likely anthracnose, a fungal infection, although septoria leaf spot can't be completely ruled out. In both cases, there is very little that can be done once the disease becomes apparent. Although unsightly, neither disease is at all likely to be fatal unless infection occurs for several years. There are fungicides that can be applied next spring when the leaves are "mouse ear" size. This should prevent reinfection. Disposing of fallen leaves will also help.

You could try and prune it to a smaller size maybe if you wish to keep it in a smaller pot, my key lime is quite small and it went through several flowering cycles this year. Citrus are heavy feeders so providing enough nutrients is key. I use granular citrus fertilizer on a monthly basis just topdress the soil, and when my plants are fruiting and flowering I use a water soluble fertilizer weekly. They also make water soluble citrus nutrients which can be diluted and sprayed on the leaves. I prefer to apply smaller amounts of nutrients more often. Also I recommend using organic fertilizer, non organic fertilizer will lead to salt build up in the pot. The picture is the kinds of fertilizer I use. In the background you can see some key limes in my fruit basket.