Nitrogen makes up seventy eight percent of the earth's atmosphere and is essential for healthy plant growth. Without nitrogen green plants would be weaker and smaller than they should be. With too much nitrogen though plants are unable to withstand frosts or heavy winds. While fertilizers contain nitrogen there are also natural sources of the element. The decay of organic material is another way that soil can become nitrogen rich. Earthworms, bacteria and decay break down the proteins in the organic matter, assisting it to decompose. Ammonia compounds, mixtures of nitrogen and hydrogen form. These compounds become nitrates, which are then consumed by nitrobacteria called azotobactors. Azotobactors produce soluble nitrates, which can be used by plants.
When using fertilizers, check the package to see which kind of nitrogen you're getting. The "N" number of the "N-P-K" formula will tell you the percentage of nitrogen by weight in the mix. Quick release fertilizers will contain nitrates so your plant can use them right away. "Slow release" indicates the ammonium form of nitrogen. Ammonium nitrate is actually a half-and-half mix of nitric oxygen (quick release) and ammonium nitrogen (slow release).
The best organic sources for nitrogen are alfalfa meal, blood meal, fish emulsion and guano.