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). Finally, transplant shock can contribute to yellowing. If new growth shows up as green, that might be the problem. Try to isolate each of these possibilities one at a time to determine the problem.
Water citrus long enough so that the water penetrates through the entire root system, both vertically and horizontally. Water should reach past the dripline, or edge of the canopy, where young roots actively absorb it. For newly planted trees, up to 1 year in the ground, water should penetrate about 18 inches for a 5-gallon tree, 20 inches for a 15-gallon, and 26 inches for a 24-inch box tree. These depths should increase gradually up to 36 inches deep over the next few years. Frequency depends on many factors, including soil type, weather condition, age of the tree, etc. As a general guideline, water new trees every 5-7 days in the hot summer months, increasing to every 14 days in cooler months. As the tree ages, these timeframes are extended.
University research shows that newly transplanted citrus trees don't need fertilizer until they've been in the ground at least 1-2 years. After that, citrus feedings are given three times per year, with one-third of the tree's annual requirement. "Desert Landscaping for Beginners," 0-9651987-3-1, Arizona Master Gardener Press, has an excellent chapter on citrus growing for the low desert, including watering and fertilizing tables that take away the guesswork.
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