Farmers in China, Japan and Korea use human waste as fertilizer. But farmers in other parts of Asia where the main crop is paddy rice, like Thailand and Malaysia, do not.
Some sources (like King 1911) argue that this was necessary in order to maintain soil fertility and support large populations over centuries of continuous use of fields. But online sources are quite vague about when the practice started – the Wiki entry on night soil mentions the 9th century in Japan, and McGarry gives the Yuan dynasty (circa 13th century). Often it’s just “thousands of years.”
Do we know anything about when precisely the use of night soil began? Presumably it originated in China. It would be interesting if we could show that this was an innovation that resulted from population pressure or some other change in the ecosystem.
References from this page: http://www.agroecology.org/Case%20Studies/nightsoil.html