Ammonia is the crucial intermediate for the production of nitrogen fertilisers. BASF, today still one of the largest chemical companies in the world, was the first company to develop a process for the synthesis of ammonia from its compounds hydrogen and nitrogen: the well-known Haber-Bosch process. Other processes were developed as well in the 1910s and '20s but these technologies have often been classified as 'imitations', neglecting the crucial role they played in the take-off of the fertiliser industry in the late 1920s and neglecting engineering contractors, companies that specialised in the design and construction of plants for the process industries. Engineering contractors diffused ammonia synthesis across the world and enabled the take-off of the fertiliser industry. This paper attempts to show the role engineering contractors can play in the chemical industry and attempts to give a more balanced account of the history of ammonia synthesis by focusing on the fertiliser industry instead of BASF.