Many concurrent inputs are necessary to grow crops, but nitrogen has a special place among them as it is commonly the most important yield-limiting factor. Its presence is irreplaceable for the synthesis of amino acids making up essential proteins needed for human growth and survival. Traditional agricultures could rely only on recycling of organic matter and on planting of leguminous crops for renewal of the nutrient. Commercialization of Haber-Bosch ammonia synthesis led to the rising importance of inorganic fertilizers. In industrialized nations their applications support diets rich in meat and dairy products-but in poor and densely inhabited countries they have become an existential necessity providing enough protein for rapidly growing populations. This importance will increase during the coming decades.