Apply fertilizer high in nitrogen such as ammonium sulfate (21%N), for blood meal (13%N) Feb June4-6 weeks apart. See Citrus for the Home Garden for the amount of fertilizer to use on young and mature trees.
http://www.mastergardenerssandiego.org/downloads/citrus%20for%20home%20garden.pdf
If you use a citrus fertilizer, follow label directions. (Avoid products that use Murate of Potash to supply potassium).