50% from (FIXED) nitrogen is wrong, though.

Fixed nitrogen, ie ammonia (for instance), is made from gaseous nitrogen and hydrogen. 3kg of hydrogen makes 17kg of ammonia. 200 pounds of ammonia (16kg of hydrogen) will make about 250 bushels of corn. 1 bushel of corn is about 370MJ. 1kg of hydrogen is 142MJ. So 2.3GJ of hydrogen makes 92GJ of corn. That’s about 3%, not 50%.

BTW, the US produces about 17 megatonnes of ammonia per year, or 3 Megatonnes of hydrogen per year. At 60% efficiency, that would require about 22 Gigawatts of average electricity, or less than 5% of average US electricity usage. The US grid is about 40% clean, so that’s still only about 10% of US’a clean electricity usage.