MODERN SOIL SCIENCE—KELLOGG 25K 
are still going on along with soil formation. The natural erosion of the 
uplands gradually removes a little of the surface bit by bit while the 
soil film settles down, and fresh minerals are added to the soil from 
beneath. With the warping of the landscape these processes are 
accelerated or retarded. 
At several of the Soil Conservation Experiment Stations of the 
United States Department of Agriculture, rates of erosion were deter- 
mined under permanent vegetation (15).2 Under the natural forest 
Exchangeable Cations (M.E,/100gms.) Exchangeable Cations (M.E./100gms.) 
pH 10 20 30 40 pH 
5.1 
4.8 
49 
4.4 
Si 
6.2 
$.6 
B. From granite 
a ed, 
Percentage of Clay 
A. From diabase 
Fiaurs 2.—A comparison of the clay content and exchangeable bases from two 
soils developed in similar environments, except for the difference in parent 
rock. The fundamentally important differences in clay content and in ex- 
changeable bases are obvious between the soils and among horizons within 
each soil (9). 
cover of the Cecil soil of the Piedmont, erosion proceeds at a rate of 
about 1 foot in 10,000 years. This value was determined on a 10- 
percent slope. Yet on a 14-percent slope of the Muskingum silt loam 
of Ohio, considerably over 200,000 years would be required to remove 
1 foot by erosion under the forest cover. 
Under a well-established grass cover, normal erosion proceeds 
slowly on the dark-colored soils developed under tall prairie grasses. 
On the Marshall silt loam near the Nebraska-Iowa line, with 9-per- 
cent slope, nearly 14,000 years would be required to remove 1 foot 
under bluegrass. On a black soil of east Texas, Austin clay with a 
slope of 4 percent, the figure is nearly 900,000 years. 
3Numbers in parentheses refer to literature cited at the end of this article. 
866591—50-——_16 
