Zaz ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
These values probably do not represent the real extremes, but rather 
relate to normal erosion. Some soils erode much more rapidly under 
clean cultivation with bad effects upon soil productivity. Through 
proper cropping systems and soil management practices, erosion of 
soil under use should be kept somewhere near the normal rate. 
Organic Organic 
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A. Miami Stit Loam- 
Undulating to Rolling 
B. Bethel Silt Loam- Flat 
Ficur& 3.—A comparison of the mechanical composition of two soils developed 
from similar parent materials and in similar environments, except for relief. 
On soils of undulating to gently rolling relief there is continually a small 
amount of erosion in the natural landscape. As the surface is thus slowly 
eroded, each soil horizon works down into the one beneath. Over a period of 
several thousand years the whole soil profile, while remaining at the same 
length, may sink into the landscape a foot or more. Thus the soil is kept 
constantly renewed with new minerals. This is illustrated by the Miami silt 
loam at the left. The soil at the right is developed from similar material on 
flat upland where there is little or no erosion in the natural landscape. The 
leached material accumulates at the surface, and clay formation and the accu- 
mulation of clay are accentuated in the middle portion of the profile, with the 
development of a claypan. Whereas the Miami silt loam is a well-drained soil, 
pervious to roots and water, the Bethel is an imperfectly drained soil. During 
wet periods excess water is held up by the claypan which is also impenetrahle 
to mast roots. Kinds of crops and soil-management practices for optimum 
production are quite different indeed, even though the soils lie side by side on 
the same farm (2). 
Percolating water gradually dissolves the minerals in the soil and the 
rock beneath. Clarke has said that this process alone reduces the 
surface of the United States on an average of about a foot in 30,000 
years (6). From a study of streams, Dale and Stabler (7) estimated 
