Citrus are usually fertilized three times a year - For oranges, tangerines, and grapefruit, use 1/3 the recommended annual amount in Jan/Feb, 1/3 in Mar/Apr and 1/3 in May/June. For lemons and limes, use 1/3 the recommended annual amount in Jan/Feb, 1/3 in Mar/Apr and 1/3 in Aug/Sept.. Newly planted trees do not need fertilizer the first 1-2 years. Use Ammonium Sulfate, Ammonium Phosphate or Citrus Food fertilizer. The amount of fertilizer needed per year depends on the age, the size, and the type of tree. For example, a medium-sized adult tree 5-6 years after planting needs 6.2 pounds of Ammonium Sulfate per year (split into three applications). Grapefruit trees 5 or more years after planting need half the amount for other citrus.