The problem with testing soil for nitrogen is that the chemical is so changeable. It evaporates readily in one form and it easily forms many different compounds with other chemicals. A simple chemical analysis simply cannot identify and measure all of the nitrogenous compounds in the soil.
Most off-the-shelf testing kits only measure the amount of nitrogen available in the soil at that exact time. Just be aware that the nitrogen levels fluctuate throughout the year. Also, once the soil warms in late spring and early summer microbial activity increases causing release of nitrogen making for different readings.