Crop production to meet the increasing food demand requires irrigation water and artificial fertilizers to enlarge the arable land area and promote unit yield. If the water supply is sufficient, irrigation water can be provided, and, notably, artificial nitrogen fertilizer can be self-sufficient and renewable through ammonia synthesis. Ammonia can be synthesized from hydrogen obtained from water electrolysis and nitrogen in the air separated from the other gases through cryogenic separation or pressure swing adsorption. Ammonia is an indispensable starting material for artificial nitrogen fertilizers and is essential for crops to improve yields. In current industries, ammonia is synthesized from fossil fuels. Then, "green ammonia," synthesized based on renewable energy, is inevitable for a sustainable future. Obviously, nitrogen in the air is versatile, and thus ammonia can also be self-sufficient and renewable if energy and water are so. Furthermore, ammonia is attracting attention as an energy carrier. It liquefies at room temperature at approximately 1 MPa, and has a high energy density in terms of weight and volume, making it suitable for storage and transportation of renewable energy. Ammonia-fueled power generation technology is being established. Therefore, ammonia has the potential to replace expensive batteries and have a positive effect on energy use.