I wouldn't bother testing for nitrogen. You can tell if you need nitrogen or not by looking at the plants. If they are a dark green and you notice growth daily, they have adequate nitrogen; on the other hand, if the plants are light green and stunted, they probably need nitrogen.

If the area you are planting is new and nothing has been cultivated, the soil will most likely need additional nitrogen either from organic materials or synthetic fertilizer. Nitrogen moves through the soil rapidly so it will probably be too low without adding a source of nitrogen.

I agree that testing for N is generally a waste of time. Even most professional soil tests qualify it or don't include it any longer because readings are so inaccurate and inconsistent. Nitrogen leaches readily as twc mentioned and is very volatile (evaporates easily) so readings can be affected by air/soil temps and even time of day.

So the best "test" is plant leaf color and growth. When in doubt it does no harm to supplement it with small/diluted weekly or bi-monthly doses. Heavy doses can affect plant productivity however.

Dave