FYI Skip, A&L Labs West does not use the same methods as A&L Labs East, so it is unlikely they all use the same methodology. You can accurately measure N in a lab however it requires special sample handling. A more appropriate value of N to lab measure would be total N, if you measure it at all. Also nitrogen contained in organic matter is far less transient than nitrate or ammonium, and a lab estimate with respect to organic-N would be more reliable. All that said, real time N availability (in particular nitrate) is best measured in the field.