To produce a bounty of fruit, citrus trees need regular fertilizing starting this month. Pick a fertilizer high in nitrogen; these trees don’t need high concentrations of potassium and phosphorus. Give newly planted trees about a tablespoon of ammonium sulfate monthly during spring and summer or apply a specially formulated citrus fertilizer according to package directions. Scatter the fertilizer in the water basin away from the trunk and thoroughly water it in. As the trees get older and larger, increase the amount of fertilizer; a four-year-old tree would get about two cups of ammonium sulfate, for example. (Consult the citrus growing guide at http://mastergardenerssandiego.org to learn how to calculate the fertilizer amounts.) Scatter fertilizer around the area under the tree’s canopy and well beyond to reach all the feeder roots. Avoid contact with the trunk. Feed every four to six weeks, until early summer; avoid fall feedings since they stimulate new growth that attracts the citrus leaf miner. Late season growth also may be susceptible to frost damage. If the trees’ new leaves have a yellow cast but the veins are still green, they may be deficient in zinc or iron. Spray the leaves with chelated zinc or iron to correct this. Overwatering also can cause these symptoms; an irrigation checkup is also in order.