Few, if any, labs will test for nitrogen. You could probably pay an extra fee for such a test, but it's really not worth your money or effort. The problem is that N is found in many chemical forms, and most of those forms are 'mobile'. Mobile means that they 'gas-off' (basically, they evaporate), they are water soluble (they run-off with rain and irrigation), and the chemical forms of nitrogen transform very quickly, and thus are not stable long enough to get accurate test levels. Urea, ammonia, nitrites, nitrates, etc, etc... when found in soil, are all forms of nitrogen. Some of these chemical forms are available to plants, some are available to microbial life, some are available to insect and invertebrate life, and some are available to fungi... and some are available to all of the above.

Let's say, for example, there is nitrogen found in fungi growing in the soil. An insect comes along and eats the fungi. Now the nitrogen is in the insect. The insect defecates, and now the nitrogen is in numerous forms... one is ammonia. Ammonia evaporates extremely quickly, so now the N is in the atmosphere. During a thunder storm, the electrical charge of lightning causes chemical reactions in the atmosphere which results in N precipitating with rain and reentering the soil environment. A plant soaks it up and uses it for a while. A deer comes along and eats the plant, and now it's again available to the many life forms of the soil web, as well as again evaporating. Without detailing every other chemical form of nitrogen, you may now get an idea of how 'mobile' nitrogen is in soil. It's always reacting, almost always being used by the soil web, and always exchanging from solids, to soluble liquids, and to gaseous chemical forms. Testing for it will never give you a reliable level of how much is available to your plants. Take a sample one day, and by the time you get back your results of the specific forms tested for, the available amounts will be completely different... even depending on the weather of one day.

Courtney, my email says you posted a response here, but for whatever reason I can't see it.

Your question I see in the notification email was...

"So how do you know if your soil needs nitrogen?"

From your previous discussions, I remember that you're mostly doing perennial and ornamental plantings. The easiest answer to your question about nitrogen is... most soils can use at least some nitrogen, but few soils need nitrogen. They way you determine if you need nitrogen is to look at your plants - nitrogen deficiencies show up in leaves and growth.

In plants, sufficient nitrogen results in lush growth and good, [usually]green colors through out the plant. N deficiencies show in spindly growth, and pale green to yellow colors starting from the lower leaves and up (of course, non-green plants won't turn green from sufficient N.) Colors are relative though, so you aren't looking for deep green colors in every plant, you are looking for the optimum expression of the plant's intended color consistently throughout the plant. If the natural color of a plant is pale green, more N won't make it darker.

The soil type that can most benefit from N additions are sandy soils. They tend to drain rapidly, and the sand has little absorbancy. Adding compost to sandy soils is the best way to improve its ability to hold on to mobile nutrients. The most sustainable way to keep N in lawns is to leave grass clippings in place, letting the soil web recycle the N.

Since N is delivered even by rainfall, a healthy soil will collect and 'store' N in the soil web. Every time a worm, for example, leaves castings behind, N will become available to the plants. When a plant drops a leaf, the soil web will consume the N and it will recycle. The healthier the soil and the corresponding biome, the more your plants will have available opportunities to take up nitrogen (and all nutrients.)

Intensively harvested Veggie gardens will usually require more N than other plantings, but even then, the same axiom is true... the healthier the soil, the more N will be stored from the larger environment and become available to plants.

So... if you have a lot of sand (as many people do in Florida,) expect to need N until you can build up a healthy soil structure. Learn what your plants are supposed to look like and be attentive to their needs. Let them tell you what they need. Composts - really, all organic matter - and the corresponding soil life that comes from a healthy biome, will store and recycle N. Kitchen scrapes, for example, are rich in nitrogen.

One last word that some find a bit 'off-putting'... in cases where gardeners are just beginning to build a healthy soil, and they still need to 'prime the pump' so to speak by adding N, many will remember that all life forms excrete lots of liquid N. Urea and ammonia are rich sources of nitrogen. In fact, there is so much available nitrogen in pure urine, it can cause a nitrogen overdose and 'burn' plants if not diluted. A good dilution ratio is 10 parts water, 1 part urine. Humans are a part of the natural environment... free, safe N is available to everyone. If you choose, just don't over do it and you'll be fine.