A little quick math here.....

15 gal of 28% would have about 3 lbs N/ 1 gallon.... so 15 gallons would have around 45 lbs of N.   Typical recommendations for garden crops is about 50 lbs N/acre, with sweet corn being one exception which requires more..... more like 75-100 lbs N/acre. 

Soooo...... you have enough Nitrogen in that 15 gallons for almost 1 acre of garden.

How big did you say your garden is? 

What vegetable crops are you growing?

And, as others have said, you don't want to apply it over-the-top as it will burn your plants. Apply it to the soil, and water it in.