Answer: 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.) Soil should be kept moderately moist (but not wet).
When leaves yellow and brown around the edges, the problem is often salt burn. Salts in the water and in fertilizer build up over time. Browning usually occurs on the old leaves first. This excess salt accumulates in the leaf edges, where it kills the tissue and the leaf dries out and turns brown. It's important to water deeply and slowly. This doesn't always happen with drip irrigation. Make sure your irrigation is penetrating through the entire root ball, not just stopping after an inch or two. Salt will buildup wherever the water stops, creating a great condition for salt burn. At least once a month, water deeply enough to "leach" or push salts well below the root zone. Frequent, light "sprinklings" or drip that doesn't run long enough allow salts to accumulate in the top layers of soil, where the roots are, which is bad news. Similar symptoms occur when too much fertilizer has been applied. Always water plants thoroughly before and after applying fertilizer to help prevent burn.
Try to isolate each of these possibilities one at a time to determine the problem.
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