If you are looking for a good read on soil testing, please consider reading this simple, 2-page article. Soil Testing is worth the Effort – by Lee Reich The best part of this article is the second page on the topic of Nitrogen.
Soil tests rarely determine nitrogen levels, even though this is the nutrient for which plants are hungriest. Nitrogen is readily lost from soil, puffing away as a gas or leaching from rainfall beyond the reach of roots. The stuff is simply too evanescent to make testing for it meaningful at any single point in time. Most soil nitrogen is locked up in organic matter, so determining the percentage of organic matter in your soil offers an idea of the amount of nitrogen potentially available. I say “potentially” because nitrogen becomes available as soil microbes chew away at organic matter; the amount that plants actually get waxes and wanes as weather conditions influence microbial growth. If your soil contains about 4 percent organic matter, you probably have an acceptable level of nitrogen. Source: http://www.finegardening.com/soil-testing-worth-effort#ixzz4OuG47wvU
This article was written from an outside perspective, but Midwest Laboratories is referenced as a laboratory to consider. If you have more additional questions regarding soil testing, please consider contacting a representative at Midwest Laboratories.