REGULATING THE WATER SUPPLY 155 



as in eastern United States, the soil is heavy in texture, 

 the exposed subsoil not productive, and the loss of surface 

 soil causes serious damage. When manure, mineral 

 fertihzer, or lime is used, much of this added material 

 remains in the plowed surface and erosion causes a direct 

 loss of this material. 



109. Causes of erosion. — In the corn-growing area of 

 the United States — that is, from the Atlantic Coast 

 westward to the 100th meridian — • erosion is related to 

 the amount of run-off water and to the condition of the 

 soil at the time the run-off takes place. In the principal 

 corn-growing States, north and west of the Ohio River, 

 erosion is not serious. The land is generally level and 

 rainfall not excessive. Also, during a part of the year the 

 ground is frozen, and in June, July, and August, when 

 about 40 per cent of the rainfall occurs, the land is in crop. 



From Ohio eastward, however, the rainfall is heavier and 

 cultivated land is more rolling, thus increasing the total 

 run-off and erosion. From the western edge of the Corn 

 Belt to the Atlantic Coast, erosion gradually increases. 

 In Kansas and Nebraska, with level farming land,' the 

 rainfall is 25 to 30 inches and the total run-off about 3 

 inches. In the North Atlantic States rainfall is heavier, 

 land more rolling, and the run-off is estimated at 40 to 50 

 per cent of the rainfall, which often amounts to a run-off of 

 20 inches or more. In the Southern States the most 

 serious erosion takes place during the winter months. 

 The soil is not frozen, is without a crop, and heavy rainfall 

 occurs during this period. 



The relation of cropping systems to erosion may be 

 grouped as follows : — 



(a) Land in grass erodes least. 



(b) Land in stubble or small grain erodes more than (a). 



