Answer: It sounds as if you are doing everything right with your tomatoes. Yellow leaves can be caused by many things including lack of nitrogen, insufficient 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 and that sounds like it could be a possibility in your case. Soil should be kept moderately moist (but not wet). It's a little early in the season for a fungal disease called late blight, but yellowed, spotted leaves can be a symptom of that disease. I'd remove the affected leaves to keep fungal spores from splashing onto healthy foliage. I hope this info helps.
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