WHAT IS BECOMING OP OUR SOILS? 221 



fering the loss of seventy-five to one hundred million dollars 

 through the agency of erosion. 



FACTORS DETERMINING EROSION. 



A remarkable study of soil erosion has been made 

 by the late Dr. W. J. McGee, who was an expert in 

 charge of soil water investigations of the Bureau of 

 Soils. According to McGee's conclusions the amount 

 of erosion which takes place in any locality is deter- 

 mined by a number of factors. First of all the quan- 

 tity of precipitation is an important factor, but not 

 quite so important as the exaggerations of precipita- 

 tion. A rainfall of forty inches a year which is dis- 

 tributed evenly throughout every month, being a little 

 over three inches per month, and this distributed evenly 

 throughout the month, might produce no erosion at all ; 

 while, on the other hand, a rainfall of fifteen inches 

 a year which should come all at once, or within a short 

 period, would produce very decided erosive effects. 



Aside from the amount and distribution of the rain- 

 fall, the contour of the land and the texture of the soil 

 are important controlling factors. In land that is 

 practically level erosion is reduced to a minimum ; the 

 more hilly the land, the greater the erosion, other things 

 being equal. A soil that is tenacious, contains plenty 

 of lime, and at the same time porous, is least subject 

 to erosion ; while a soil that is sandy, devoid of humus, 

 and containing little lime, is most prone to erosion. 



McGee calls attention to the amount of precipitation 

 in the United States. In the mainland of the United 

 States, exclusive of Alaska and the insular possessions, 

 the mean annual rainfall, as shown by the reports of 

 the Weather Bureau, averages about thirty inches. The 

 total quantity of water precipitated amounts to 215,- 



