Question
Why is it that the preferred source of nitrogen is nitrate nitrogen?

Answer
Scott, good question. Nitrate nitrogen is the inorganic form which is the form most easily utilized by plant tissue to produce proteins of various kinds for plant growth.  Other forms of nitrogen, such as that found in urea, require digestion into simpler compounds that can be used by plant tissue. This means that it might take weeks to months before the nitrogen becomes available for plant growth. Obviously, in nature, it is usually these more complex forms from dissolved organic solids that form the basis for growth.  This suggests that when using fertilizers you should use quite dilute solutions.  Were you to begin feeding when the plant starts new growth, you want the nitrogen to be available immediately.