At the end of the First World War, the French government seized the opportunity to acquire the chemical processes of the German firm BASF, including the Haber-Bosch process. This patent made it possible to synthesize nitrogen from the air and thus produce nitrogen fertilizers in large quantities. French industrialists, however, refused to acquire these patents, and to make up for this lack of private sector involvement, the French Parliament decided in 1924 to create a national plant (ONIA), which became the first state-owned plant to be exposed to market competition. The intention was for the ONIA to supply the army with nitric acid in times of war, and, in peacetime, to sell fertilizers at the lowest possible prices in order to curb the monopoly of the private industry cartel. The purpose of this article is therefore to study the establishment and organisation of the French market for nitrogen fertilisers during the inter-war period by raising a number of questions about the ambiguous and complex relations between the state and private industry in this strategic sector. Why was the state policy initiated with the ONIA not successful at first? From 1927-1928, once the ONIA was operational, why and how did the public and private players jointly organise the marketing of fertilisers even though their interests were partially divergent? From the economic crisis of the 1930s onwards, how did the regulation of this mixed market evolve and how were public/private tensions overcome? In the French case, why did French producers leave the international cartel very early on in favour of state protectionism? And finally, to what extent can it be said that this “managed economy” framework succeeded in satisfying all the players in the French nitrogen industry?