<Dd>... all progress in capitalistic agriculture is a progress in the art, not only of robbing the labourer, but of robbing the soil; all progress in increasing the fertility of the soil for a given time, is a progress towards ruining the lasting sources of that fertility . The more a country starts its development on the foundation of modern industry, like the United States, for example, the more rapid is this process of destruction . Capitalist production, therefore, develops technology, and the combining together of various processes into a social whole, only by sapping the original sources of all wealth--the soil and the labourer . </Dd> <P> One of the most widespread occurrences of soil depletion as of 2008 is in tropical zones where nutrient content of soils is low . The combined effects of growing population densities, large - scale industrial logging, slash - and - burn agriculture and ranching, and other factors, have in some places depleted soils through rapid and almost total nutrient removal . </P> <P> Topsoil depletion occurs when the nutrient - rich organic topsoil, which takes hundreds to thousands of years to build up under natural conditions, is eroded or depleted of its original organic material . Historically, many past civilizations' collapses can be attributed to the depletion of the topsoil . Since the beginning of agricultural production in the Great Plains of North America in the 1880s, about one - half of its topsoil has disappeared . </P> <P> Depletion may occur through a variety of other effects, including overtillage (which damages soil structure), underuse of nutrient inputs which leads to mining of the soil nutrient bank, and salinization of soil . </P>

Which layer of soil is the most fertile