The effect of cultivation and the growth of crops upon the nitrogen and humus content of soils has been studied by various investigators, both in America and Europe. In general, the results of the various investigations indicate that cropping and cultivation are very destructive of the organic. matter and the nitrogen of the surface soil.
Stewart, Robert, "Bulletin No. 109 - The Nitrogen and Humus Problem in Dry-Land Farming" (1910). UAES Bulletins. Paper 60.