So anyhow, if you're going to buy a fertilizer, and yes there are ones especially made for Avocado, any fertilizer however should consists of three basic substances it's actually their percentage that usually changes from plant to plant.
Nitrogen helps to grow the leaves (mainly) while Potassium increases the efficiency of the growth. And Phosphorous is the key ingredient involved in growing the roots of the tree. It basically acts as what which promotes the cell division & the development of new tissues of the tree as well. So basically, out of the three, it's the Phosphorous that's the most important substance (that's not to neglect the other two by any means!). If the grow efficiency or the speed of the growth is slow and the tree leaves (the body included) have an abnormal dark-green color, that's usually an indication of a lack of Phosphorous.
For young (small) Avocado plants, it is recommended that you add 0.5 to 1 pound of Nitrogen per year. Also make sure when you buy a fertilizer, that it also includes Zinc as it's quite important to the Avocado tree growth as well (you should be able to find it in under ingredients). And just as with adding any fertilizer, make sure to water it often so the ingredients are broken down (have you known? Among other things, water breaks thing down) and are mixed to give the proper nutrition to your Avocado tree.
So basically, a fertilizer that's created specially for Avocado trees should take care of the percentages of the substances that are included.
Now I'm not going to spill all the details here. For instance, there are few things that you should consider before deciding to plant an Avocado tree. So if interested, you can get further information about planting Avocado (the proper climate, how and when you should add the fertilizer etc) from the below sources: