Final answer:
Nitrogen, located in the fourth period of the periodic table, is used in making fertilizer, and its most common fixed forms for this use are ammonia, ammonium nitrate, and urea.
Step-by-step explanation:
The element in question from the fourth period of the periodic table used in making fertilizer is nitrogen (N). Nitrogen is a vital macronutrient for plants, essential in the synthesis of proteins, DNA, chlorophyll, and some vitamins. It is not readily available to plants in its atmospheric form and must be 'fixed' into a usable form like ammonia. The Haber-Bosch process enables the production of ammonia by combining atmospheric nitrogen with hydrogen, typically sourced from natural gas, under high-pressure and temperature conditions. Ammonia is the feedstock for nitrogen fertilizers such as ammonium nitrate (NH4NO3) and urea (CO(NH2)2).