Grapefruit are citrus tree fruits related to the pummelo that were created by a mutation and are now produced in many dozens of hybrid cultivars. Grapefruits thrive in frost-free subtropical and tropical climes. Like many other citrus varieties, grapefruit trees are not heavy consumers of fertilizer. A light application of fertilizer several times during the growing season is sufficient to keep the plants growing and producing fruit harvests.
Fertilize your grapefruit tree three times during the growing season each year in February after new growth emerges, in June and again in October.
Apply a powdered or granular ammonium sulfate nitrogen fertilizer with a guaranteed analysis of 20-0-0.
Sprinkle 1/3 cup of the ammonium sulfate around the drip line of the grapefruit tree three times per year when the tree is a year old. Increase this to 2/3 cup three times a year at 2 years old, and increase to one cup three times a year at 3 years of age.
Fertilize your mature grapefruit trees following this aged-based formula so that a 6-year-old tree would receive two cups of ammonium sulfate three times per year for a total of six cups of fertilizer for the year.
Water the fertilizer into the soil until the soil is drenched and the nutrients begin to soak into the soil.