alexs  said:
Perhaps the soil-depletion mechanism works in the downfall of
civilizations. Once a trace element is depleted, the masses
suffer declining vitality, perhaps allowing epidemics to spread.

Yes!  If you read NUTRITION & PHYSICAL DEGENERATION  by Weston A. Price,
DDS.  You will learn about how crucial the mineral content of the soil is
the health of the people and animals feed off of it.  He explains a lot
about the carrying capacity of the land, relative to the people and their
health.  He studied 13 native population groups who were isolated from the
foods of commerce (packaged, processed, refined foods....).  When he
analyzed their food it was amazing how many times more vitmains, minerals,
fat soluable activators (such as Vit. A & D), they ate compared to modern
people.  He comments at length about the analysis of their food and of their
physical, mental, and moral health.

His book is a classic and I urge you to get it.  The pictures are
incredible.  Since the book is now available in paperback, it is more
affordable than it used to be.  Dr. Price's video is also great. I have
shared it with many people.  The video costs about $35 but if you knew what
went into making it from slides and prints from the 1930s and you saw what
it offered....you'd consider it a bargain. It's a documentary of Price's
work.  The Price Pottenger Nutrition Foundation sells these.

Back to work,
Rachel