Fritz Haber
Haber figured out how to make ammonia using nitrogen and hydrogen.  This was important because ammonia can be used to make fertilizer that makes crop plants grow.  Before this, people dug nitrates out of the ground or had bird feces shipped all over the world to farmers, and the bird feces was being used up faster than the birds could replace it.  After Haber learned to make ammonia, people could make as much fertilizer as they wanted.  They said Haber had made "bread from the air."