Last year, I had a container Meyer Lemon that died. Before it succumbed, the dropped leaves looked like this. Sorry for the giant picture:

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What do you think caused it? Nutrient defieciency or overwatering? Something else?

I have another tree now, and some of the older lower leaves have a little yellowing at the very tips:

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Overall, I think it is healthy. I'd like to keep it that way. I started fertilizing once a week with Foliage Pro, 1/4 to 1/2 teaspoon. I want to make sure the yellowing tips aren't a sign of fertilizer burn. Or that I am letting it get too dry. I can see roots at the drainage holes, but I am waiting until spring to repot.

Here is what the tree looks like:

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Thanks for your help!