I was, just earlier this week, looking up fertilization recommendations for several plants that I have, including lilacs. We have several 2-4 years old, that have not bloomed yet so that is one thing that interested me.

Here is the information I found.

Use a low-nitrogen, high phosphorus fertilizer, such as a 5-10-10 or 5 -10-5 blend, around the drip line of the shrub in early spring. Use the least recommended amount from the label of the fertilzer. Till into 1-2 inches depth. Water immediately. Apply only after the old flowers have dried up.

Otherwise, I also heard 1 cup of epsom salts around the drip line once or twice a year, for lots of blooms.

OR, 1 gal water plus 2 oz Epsom salts and pour on lilacs when they are dormant (late fall to early spring).

Mulch around base to keep soil moist and cool.

Hope this helps!

Sometimes when a lilac doesn't bloom and it is getting enough sun it means it is planted too deep. I have known more than one person who has dug out and lifted their non-blooming lilac slightly higher than it was planted before and the next year it bloomed. This is hard to do, however, if your lilac is large.
Deb