the base pH that the soil is set to is simply the pH that the soil will drift to when dry. This is why it is important to come in at the low end of the soil range, ie, 6.3 pH so that your plants will see the entire range of pH so as to be able to pick up the lesser mobile elements. N is available over a wide swath of pH and 99.9% of the time pH is not a problem in its uptake. Unless your soil is way too acidic to support life or way to base (which a commercial soil would never be) Nitrogen is going to be mobile. It is the easiest element besides water and c02 to uptake into our plants and rarely do we see N deficiencies, and almost never because of pH.