Citrus trees problem

I have 5 sweetlime trees and one lemon tree.I was wondering why the leaves on these trees are yellow? The branches on the older tree start to die off.the lemon tree is the youngest one and the leaves barely started to turn yellow…

The 1st pic is lemon and rest is sweetlime.soil is clay.

Feed them Nitrogen quarterly. Make sure that the first of the 3 numbers (the NPK) on the product is much larger than the others. However: do not use products labeled as lawn food. Follow the dosage instructions on the label.

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