my grapefruit tree is from a seed. 15 years old, 15 feet tall, this is the first year it has had fruit (about 20). what type of fertilizer should I use and how often. I live in Charleston SC.

Congratulations, that is a long time to wait for fruit!

As for fertilizer, the general recomendation is for a fertilizer with 5-1-3 ratio (N-P-K), at least that is the ratio the tree will use, however, for inground citrus the ratio is not as important as for container citrus. You may be able to find a fertilizer made for citrus at someplace like HD, however since you are outside the normal citrus area --maybe not. You also need to use a fertilizer with trace minerals or add trace minerals separately. Since your tree is inground, you do not want to fertilize between Sept and March. Apply the fertilizer in at least 3 separate applications between March and Sept.

Cdoc