Fertilizing your orange, lemon and grapefruit trees will keep them healthy, green and productive. Purchase a high-nitrogen liquid or granular fertilizer.
Liquid fertilizers are easiest to use on citrus trees that grow in containers.
Use granular fertilizer on trees planted in the ground. Water the tree before applying the fertilizer. Apply fertilizer evenly to the soil beneath the tree's canopy, at rates recommended on the label. Water the tree after fertilizing to wash nutrients into soil. Repeat the process every four to six weeks from February through August for trees growing in the ground.