Nitrogen is an element, and it’s present in vast amounts in the atmosphere. Your fertilizer almost always contains nitrogen taken from the air. It finds its way into fertilizer in a couple of ways. Synthetic fertilizers are manufactured using something called the Haber-Bosch process which draws nitrogen from the air and converts it to ammonia. It’s an energy-intensive process that also uses a lot of fossil fuels, but without it, there's be a lot of hungry people. Around half the worlds food production relies of fertilizer produced this way. Its likely that you’ve used fertilizer produced with this technique. Organic fertilizers on the other hand exploit the nitrogen already in plants. While some plants like legumes and clover can draw nitrogen from the air for their growth, most take it from the soil. They use it as expected to build the leaves, stems, and branches of themselves. In the fullness of time they die, and that nitrogen can be accessed once more. When composted, the decomposing matter releases its nitrogen into the soil nearby. Other organic fertilizers like manure and livestock products let animals do the work for us in processing that plant-based nitrogen. Urine and feces are rich with nitrogen and other minerals taken from an animal’s diet, and even worm castings and the like are the byproducts of tiny creatures going about their lives. Once they too shuffle off this mortal coil their bodies decompose much like plant matter does, releasing nitrogen again. Finally, there are some sources of nitrogen that don’t involve these natural cycles or modern chemistry. Nitrogen in some fertilizers may come from mineral sources, mined from nitrate deposits of one type or another around the world.