Answer: Yellow leaves can be caused by many things including lack of nitrogen, light, water-logged soil (plant roots need oxygen to thrive), dry soil, or iron deficiency. 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.) Soil should be kept moderately moist (but not wet). Hope this gives you some clues as to what the problem might be.
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