how to test for nitrogen?

So my soil test did not test for nitrogen, because apparently that changes all the time. OK, but how do I know that there's enough nitrogen before the leaves turn yellow? How do I know that the compost that I produce has something close to the ideal C/N ratio? Does someone has a test kit? How exact are they or other measuring "tools" like the colour of the leaves chart...?

I think the nitrogen demand or requirement might change even between two different kinds of plants growing together.

I don't mind just estimating and adding a little nitrogen if I think the plants are growing fast and producing lots of leaves, or if some lower leaves look yellow. I've found that coffee grounds or diluted urine both have a very fast result. But I'm only growing a home garden, nothing commercial or large, so I can afford to fiddle around. Might not work for something larger or with higher standards of perfection.

The test for Nitrogen requires a special apparatus called a Kjeldahl digestion apparatus. It also requires a titration setup to get the figures for the calculation of quantity of nitrogen.
However, this only tells you the total Nitrogen available in the soil, not the usable nitrogen that the plants can take in, so it really isn't such a great test.
If you are growing "heavy feeders" such as corn, sorghum or other grains, you can use spent coffee grounds, grass clippings, moringa leaves or plant clovers or any other nitrogen fixing plant as a mulch layer.
Another method is to use a protein food such as fish, or fish meal, as an amendment to the soil just under the surface or at planting time.

If you are making your own compost you can add any of the above nitrogen supply plant  materials and your compost will add sufficient nitrogen to the soil.

Redhawk

Thanks that is clear information! The nitrogen sources though are difficult to get by. And apart from my own coffee my guess is in our green alternative town all the cafes already have someone to pick up the grounds.  But it would be worthwhile asking.But we talked about roadkill in another thread and I thinkt this is a awsome source not only of nitrogen.