That is not a nutrient deficiency. On citrus trees they are very easy to diagnose: green veins on a yellow leaf means iron deficiency, while old yellow leaves and green new leaves mean nitrogen deficiency. I don't see either. This doesn't mean citrus trees don't need fertilizers, far from it. In fact citrus farming is where the present generation of leaf spray fertilizers were first widely used with success. I suggest you check into those for the future: I've been using them for over a year now on other crops and they are amazing products.
This is a watering issue: in the specific the tree is dying of thirst. People assume that just because citrus trees are grown in very hot locales like Andalusia and Sicily they are drought tolerant, but they are just the opposite: they need lots of water to thrive and lots and lots of water to do well. And in the top picture it seems to me the potting compound is as dry as bone.
Finally another note: differently from ornamentals like camellias, citrus trees don't like small containers. Get a larger container for it.
