Nutrients are elements that provide nourishment to plants, animals and other living organisms. Nutrients are found in nature, sometimes in their elemental state and sometimes combined with other elements. Unlike fuels, nutrients are never exhausted: they continue to exist in cycles and can be used over and over again to sustain life. However, high-concentration deposits of nutrients are finite.

Most fertilizer nutrients are mined from the ground, but nitrogen is primarily captured from the atmosphere, which is 78% nitrogen (although in its chemically and biologically unusable gaseous form). Eighteen elements have been shown to be essential for higher plants, but not all elements are required by all plants. Carbon, H and O are obtained from the atmosphere and water, and are not considered mineral elements. The remaining essential elements for plants can be divided into three groups based on average concentrations in plants: primary macronutrients, secondary macronutrients and micronutrients.

Plants cannot absorb the organic forms of nutrients, which must be mineralized before they can be used by crops and other plants. Fertilizers generally provide nutrients in their inorganic, or mineral, forms which are readily available for use by plants.

Responsible farmers replenish nutrients extracted by crops

Nutrient cycles exist at different levels, ranging from the local area to vast global systems. Some nutrients, like nitrogen, are extremely mobile in the environment and are involved in large global flows. Others, like phosphorus tend to remain in local cycles, unless they are transported physically (e.g. through the movement of crops or as a result of erosion). The management of each nutrient must take these particularities into account.

Gains and losses of nutrients in undisturbed ecosystems are roughly in balance so that continued biological growth depends on the cycling of nutrients between the biomass and the organic and inorganic stores. Removing or harvesting portions of the biomass from the ecosystem without replacing the nutrients contained in the harvested product ultimately depletes one or more of the nutrients. Nutrient removal by crops is the largest factor accounting for nutrient depletion from well-managed crops, and fertilization generally aims to replenish these nutrients.

Only a small proportion of the nutrients found in human and animal wastes are returned to farmland for recycling. Where possible, farmers use manures and other local sources of nutrients, but these sources are not adequate to grow enough crops to meet the needs of today’s population, so farmers supplement them with fertilizers.

The fertilizer industry and modern cities developed together

During the Industrial Revolution of the late 18th and early 19^th centuries, farmers needed to raise agricultural productivity to feed the rapidly expanding urban population. Crop rotations and farmyard manure no longer sufficed to ensure adequate crop nutrition. The production of superphosphates from the 1840s and the use of potash fertilizers from the 1860s helped. Nitrogen became available as Peruvian guano, Chilean saltpeter and sal ammoniac (extracted from coal) in the second half of the 19^th century. However, these sources of nitrogen quickly proved inadequate. The chemical breakthroughs made first by Fritz Haber and then by Carl Bosch that led to the modern process for ammonia synthesis – the source of virtually all nitrogen fertilizers today – was considered so important that it earned two distinct Nobel Prizes for Chemistry (Haber in 1918 and Bosch in 1931) .

Food security and economic development became major political priorities after World War II, and the use of fertilizers increased significantly to support these goals. Scientists calculate that today the lives of around half of humanity depend on Haber-Bosch nitrogen. (Erisman et al, 2008).

During the second half of the 20^th century, manufactured fertilizers helped to feed billions of people, to raise incomes and to protect farmland from degradation and forests from the plough.