CHAPTER XXVII 



SOIL EROSION 



EROSION is the removal of soil material by air or water in 

 motion. The work of water alone will be considered in this chapter. 

 The National Conservation Commission 1 states that " on the basis 

 of estimates received from 30,000 farmers, representing every 

 county in the United States, 10,622,000 acres of farm land have 

 been abandoned, and that 3,888,000 acres, or 0.2 per cent, have been 

 devastated by soil erosion." Large areas in the states from Penn- 

 sylvania to Georgia and westward, including Kentucky, Tennessee, 

 Alabama, Missouri, Arkansas, Louisiana, and Mississippi, are sub- 

 ject to serious damage by erosion. Even such prairie states as Illi- 

 nois and Iowa suffer loss in this way. As an average of the sixty- 

 one -counties of Illinois, of which a detailed soil survey has been 

 made, it has been found by actual measurement of the soil areas 

 that about 17 per cent is hilly and subject to serious erosion. 



Cause of Erosion. Erosion occurs whenever rain falls on un- 

 protected- sloping land so rapidly or in such quantities that the soil 

 cannot absorb the water as fast as it falls. The same is true of the 

 melting snow. Only that water lost from the surface the run-off 

 causes erosion. The run-off depends on (1) the slope or topography 

 of the land, (2) the texture and structure of the soil, (3) the vege- 

 tative covering, and (4) the character of the precipitation. 



1. Effect of Topography. The run-off from mountain to- 

 pography is from one-third to three-fourths of the total annual 

 rainfall when it varies from 15 to 40 inches. Newell estimates the 

 run-off from the basin of the Savannah river to be 48.9 per cent 

 (eight-year average) of the annual rainfall; 56.5 per cent from the 

 Connecticut valley (13-year average), and 53 per cent (six-year 

 average) from the Potomac basin. Greenleaf places the loss from 

 the broad level to undulating basin of the Illinois river at 24 per 

 cent, and Leverett 2 gives 21 per cent as the amount of run-off from 

 Illinois as a whole. The run-off of the United States as a whole is 

 estimated at one-third of the annual rainfall. 



2. Texture and Structure of the Soil. Coarse soils absorb 

 a much larger proportion of the rainfall than do the fine-grained 

 ones. The rate of absorption depends on the size of pores, not on 



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