Answer: Citrus are heavy nitrogen feeders. If the older bottom leaves are yellow, but new growth is green, it's usually a lack of nitrogen. If new leaves are yellow, with green veins, it's usually a lack of iron. (Lack of nitrogen is a more common problem than lack of iron.) Yellow leaves can also be caused by insufficient light, water-logged soil (plant roots need oxygen to thrive), dry soil, and shock from being moved from one environment to another.
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