When US government started recruiting people to kill them off in World War One they observed that people from certain regions had goiters. They mapped out the regions these people came from. They also found out people who ate food grown in iodine deficient soils were about 15 IQ points lower.

As you will notice, these mineral deficient soils correspond almost exactly with the glaciation patterns. the glaciers were 2 miles thick and as they pushed across the ground they scraped all the topsoil off. all the nutrients that are formed into organic , absorbably compounds are all in the top few feet of soil

Most people don't know but all these big developers now scrap all the top soil off the ground, most of the land in all the suburbs now have no topsoil.

But the germans were big fans of US eugenics studies and adopted US eugenics ideas and studied this, found the roles of these trace elements in intelligence. Copper, selenium, manganese, magnesium etc, etc in the soils was studied, this is why we have Centrum vitamins that have all these trace minerals and such. Those minerals in the vitamins are not organic forms though, they are not absorbed at very high rate.

but it is interesting that the North has very deficient soil white southern US has ancient top soil still in tact.