While nitrogen isn't needed in the same amounts that the others are, it is essential for DNA, the molecule that stores genetic information, and for proteins, the molecules that make up the muscles and catalysts that give our bodies their shape and that allow them to function in real time. Atmospheric nitrogen is converted to the usable forms by lightning or by the action of nitrogen fixing bacteria in the soil or in plants. Nitrogen in living matter is converted back to ammonia by decomposition.
Some nitrogen is released by the erosion of nitrate-rich rocks.
This biogeochemical cycle dictates the amount of nitrogen available in soil for plants. This movement of nitrogen between the atmosphere and soil, water, and living things is a biogeochemical cycle, and the rates of movement dictate the amount of nitrogen available to plants. Until the turn of the century, the nitrogen actually available for plant growth limited farm productivity to an amount sufficient to feed 5 people per hectare. Then Fritz Haber and Carl Bosh invented a method of converting N2 to NH3 by combining nitrogen and hydrogen gases at a pressure of 200 atmospheres and a temperature of 500 C. Between the 1940s and the 1990s consumption of ammonia fertilizer increased from 10 million to nearly 80 million tons per year. This artificial fertilizer doubled the amount of nitrogen available to plants, and significantly increased the productivity of the average farm.