Posted 11/18/2012 20:15 (#2704752 - in reply to #2703892) Subject: Re: the book 'Dirt' by David Montgomery
-- is a good starting place on this topic. In summary, every civilization thus far has destroyed its soil in about 1,000 years, give or take, resulting in collapse of that society.
Highly mechanized tillage (more intense tillage, and deeper) appears to be shortening that lifespan of cropland considerably. Some parts of eastern USA have eroded their soils to bedrock in just a couple centuries.