Ok. This is totally my territory.
Ammonium nitrate is the bomb. Pun intended. It's a shame it has the stigma it does, because it is a boon to the commercial farming industry. There is no other shippable solid that carries such a high % useful N.
It has a strong concentration of N, chemical formula NH4NO3. It has an N-P-K of 34-0-0.
This means that for every gram of NH4NO3 you add to 1 liter of h20, you get roughly 340 ppm of Nitrogen.
So, for one gallon, add roughly 4 grams, a little less than 1 teaspoon, per gallon to raise the total N by about 340-360 ppm, or half that your first feeding to monitor results.
If you go full bore, roughly 100-120 ppm of N will be available immediately as Nitrate N- almost instant greenery for N deficient plants.
Make sure you also supplement with the other macros, paying special attention to also adding extra Ca and Mg.
I sell great additives and amendments, too, if ya need it! This is what I do.