During the first two or three years, your tree should be fairly heavily fertilized so as to make maximum early growth. The following rates be used during the pre-bearing years:

Formulation to be applied per 1,000 gallons of irrigation water.

2 lbs. of dry ammonium nitrate or 1/3 gallon of ammonium nitrate solution 20-0-0
1 lb. dry potassium nitrate
1 pint 75% food grade phosphoric acid

NOTE : Dosages are given on the basis of the quantity of water applied to the tree. The most effective way to apply the fertilizer is probably by proportional and continuous addition of the fertilizer to the irrigation water. Since this is frequently impossible, the next best thing may be to add it during alternate irrigations. Be sure to double the amount per application in such a case. Remember to be careful not to apply all of the fertilizer in one batch while irrigating, but find some manner in which to spread out the application during a large part of the full irrigation cycle.

If you are unable to add the fertilizer to the water, or estimate the correct amount from the foregoing, you may sprinkle a tablespoon of nitrogen-bearing salt (ammonium nitrate, urea or such) over the root area and water it in thoroughly. Repeat every three or four weeks. Take care not to concentrate it in one area. Increase the dosage gradually according to the increasing size of your tree. Apply the plant food around the drip line, or in the path of irrigation water.

A fully bearing orchard is usually fertilized at the rate of 1 to 1 1/2 lbs. of actual nitrogen for an average sized avocado tree (one that has a foliage diameter of about 20 feet). This can be taken care of by sprinkling 1 to 1 1/2 lbs. of dry ammonium nitrate on the ground around and beneath the skirts of the tree two or three times a year. Wash the fertilizer into the ground with a good soaking (2 inches/5.08 cm of water). Early spring, summer and fall are good times for fertilizer application, as the avocado roots will be active then.

Source : http://home.att.net/~oc_crfg/Plant_Avocado_Trees-Brokaw.htm

Fertilizer mixtures containing 6 to 10% nitrogen, 6 to 10% available phosphoric acid, 6 to 10% potash, and 4 to 6% magnesium give satisfactory results with young trees. For bearing trees potash should be increased to 9 to 15% and available phosphoric acid reduced to 2 to 4%. Examples of commonly available fertilizer mixes include 6-6-6-2 [6 (N)-6 (P 2 O 5 )-6 (K 2 O)-2 (Mg)] and 8-3-9-2 [8 (N)-3 (P 2 O 5 )-6 (K 2 O)-3 (Mg)].

http://edis.ifas.ufl.edu/MG213