Key lime, Mexican lime and West Indian lime are all synonymous names for a cultivar of Citrus aurantiifolia, according to Texas A&M University. Key limes are small trees with a bush-like habit and spines along the branches. Key lime fruit are smaller than most other cultivars and reach roughly 2 inches in diameter at maturity. Like most citrus, they benefit from a regular feeding regimen of nitrogen rich fertilizer throughout the year.
Calculate and measure the proper amount of fertilizer to be applied each year by multiplying one cup times the age of the tree in years; hence, a 12-year old tree would need 12 cups of fertilizer in a year.
Divide the yearly amount of required fertilizer into three equal doses to be applied in February, May and September. Thus, the 12-year old tree would receive four cups of fertilizer in February, four more cups in May and the final four cups in September.
Don garden gloves and cast the appropriate amount of fertilizer evenly around the root zone of the key lime tree starting at least 6 inches from the trunk and extending roughly a foot past the outer drip line of the branch canopy.
Water the fertilizer into the soil to drench the soil at least 12 inches down so that the root zone is hydrated with the nutrient laden water.
Store your dry, granular or powdered citrus tree fertilizer in its original sealed container in a dry area with low ambient humidity below 18 percent to prevent activation or caking. Limiting exposure to moisture between uses makes the fertilizer easier to pour, measure and use.