Colourless, odourless and tasteless gas (N = nitrogenium) which, at 78% by volume, makes up by far the largest proportion in the air. The German name reminds us that nitrogen "smothers" (extinguishes) flames. Among other things, the nitrogen compounds amino acids, alkaloids, ammonium (compound with hydrogen, most common starting material for fertilisers), enzymes, nitrates and saltpetre are of great importance. 95% of the nitrogen in the soil is in the form of organic compounds (Norg) in humus formed from dead plant and animal material, in living root mass and in soil organisms. A maximum of 5% is mineral-bound nitrogen (Nmin) already available to plants in the form of ammonium (hydrogen compound) and nitrate = saltpetre (oxygen compound). However, the organic nitrogen compounds are not available for higher-value plants. They must first be converted by bacteria into ammonium and nitrate, which thus play an important role in plant growth. The roots prefer to take up nitrates, which are responsible for building up cell protein and chlorophyll during photosynthesis.