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Nitrogen fertilizers, providence turned poison

In 1909, the German chemist Fritz Haber managed to combine nitrogen in the air with hydrogen by synthesizing ammonia (NH3). One chemical reaction, among others? Not quite. This revolutionized agriculture by allowing yields to be doubled or even tripled. For many specialists, the invention of nitrogen fertilizers made it possible to feed the planet's population, which in the 20th century grew from one and a half billion to more than six billion inhabitants. This, at first glance, brilliant discovery earned its author the Nobel Prize in Chemistry in 1918 - a controversial award, as Haber was also involved in the design of the combat gases used in the trenches. This researcher's work from a Jewish family also led to Zyklon B's development and a fatal pesticide used twenty years later by the Nazis in the extermination camps.

Feeding plants is a paradox. While the air is mainly composed of nitrogen (78 %, against 21 % oxygen), they are unable to draw this essential element for their growth. It is primarily found in the soil, in the form of nitrate (NO3) or ammonia (NH3). They can then assimilate it thanks to its mineralization by bacteria, humus, and other organic matter: crop residues, manure, compost, etc. Since Haber's invention, a few fertilizer bags have provided all the nitrogen necessary for the plants and improved yields. No need to haul tons of manure or compost; no need to grow legumes rich in nitrogen ...

Nitrogen fertilizers, providence turned poison - farm landscape with clouds

Over the past century, the inexpensive production of reactive nitrogen that plants can use has completely revolutionized agriculture. In the 1960s, it formed one of the four pillars of the "green revolution": the selection of high-yielding varieties, pesticides, irrigation, and chemical fertilizers. This revolution was unanimously hailed as a great success. But, first in industrialized countries, then in developing ones, the increasing use of synthetic nitrogenous fertilizers has had effects that almost no one had anticipated.

Series of deleterious effects

Very quickly, the farmers understood that the addition of nitrogen to the crops by animal excreta (manure, slurry) and by legumes was no longer necessary. So why make it harder to raise cows or sheep and graze them? Many of them have therefore gotten rid of them to concentrate on crop production, in particular cereals. But, as it was also necessary to produce milk and meat, the demand for which was increasing rapidly, other farms devoted themselves to breeding, the most productive operating installs, without leaving the barn, and replacing fodder with grains or oilseeds.

In a few decades, the European agricultural landscape was radically transformed. In France, in the Center or the East, cereal regions without livestock resort to highly mechanized agriculture, making massive use of chemical nitrogen fertilizers. In Normandy, Brittany, Denmark, and Bavaria, livestock farming is increasingly industrialized, with enormous animals' concentrations. Farms with more than a thousand cows are becoming commonplace in several European countries, such as pigsties producing tens of thousands of pigs per year or breeding hundreds of thousands of hens. This development is a direct result of Haber's invention, rightly considered the most important in the history of agriculture - some even say history.

This upheaval, logical in a short-term economic vision, produces a series of deleterious effects, both in terms of health and the environment. Many ecological and health problems posed by modern agriculture emanate from the synthesis of nitrogenous fertilizers, or rather from the misuse which is made of them.

First problem: the organic matter content of the soil decreases in the regions of arable crops due to the lack of organic fertilizers and rotations, including crops that naturally enrich the soil with nitrogen and organic matter, such as alfalfa. High yields are still possible, but they tend to level off or even decline despite synthetic nitrogen reinforcement in some regions. Besides, the water holding capacity of soils and the rate of water infiltration decrease, which increases the risk of erosion by runoff and flooding.

Besides, pests and diseases are on the increase and require more and more pesticide treatments. Nitrogen fertilizers are not the only cause, but they contribute by the disappearance of long rotations, which interrupt the reproduction cycle of pathogens and insects and increase the