The source of nitrogen in your fertilizer affects the pH of the medium you are growing in. The standard of measure of how acidic/basic a source is, is calcium carbonate, a common water mineral. Calcium carbonate is the major contributor of water alkalinity, the capacity for water to 'soak up', or buffer, acidity in water and lead to a high pH.
Note that it is much easier to lower the pH of water through the acidity of a given nitrogen source than it is to raise it, as nitrates are less basic than ammonium is acidic.
Potential acidity = # of lbs of calcium carbonate needed to neutralize acidity of one ton of source.code:
N Source Potential acidity Potential Basicity
Ammonium sulfate 2,200 0
Urea 1,680 0
Diammonium phosphate 1,400 0
Ammonium nitrate 1,220 0
Monoammonium phosphate 1,120 0
Calcium nitrate 0 400
Potassium nitrate 0 520
Sodium nitrate 0 580
Potential basicity = one ton of source has same effect as this many lbs of calcium carbonate.