If leaves are yellowing on a citrus tree, it may be a sign of nutrient deficiency, but it may not be, depending on which leaves are yellowing.
Older leaves yellowing are quite normal as long as it’s only happening to a very small number of the very oldest leaves. Evergreen trees such as citrus will drop their oldest leaves and replace them over time, but it happens with so few leaves it’s barely noticed. Before they drop their leaves, they extract all the mobile nutrients (the ones they can move out of the leaves) to redirect into new growth, causing the yellowing of the leaves. One of the mobile secondary macronutrients is Magnesium (Mg), which is a key component in the green pigment chlorophyll which plants use for photosynthesis. Without the green pigment chlorophyll, leaves turn a yellow colour.
If young leaves are yellowing along with older leaves, and a large number of the leaves on a citrus tree are yellow, it’s a sign of nitrogen deficiency.
Citrus trees are heavy feeders and need to be fed with a good quality balanced fertiliser, so if they run out of nutrients, they start sacrificing old leaves to be able to put on new growth, and when this happens, a large amount of older leaves will start to turn yellow while new green growth emerges.
By feeding citrus trees at the beginning of spring and autumn (September and March in the Southern hemisphere, the reverse in the Northern hemisphere), citrus trees stay healthy and productive and nutrient deficiencies are avoided.