NATURE OF SOIL 



233 



as the sixth power of its velocity. This means that doubling 

 its velocity increases the carrying power sixty-four times ; 

 trebling its velocity increases the carrying power seven hun- 

 dred and twenty-nine times, and so on. 



The texture of the soil has much to do with erosion. 

 All soils that permit ready absorption of water, as from 

 rains or melting snow, leave less to run over the surface, 

 and so are less subject to erosion than dense clay soils. 



Effects of water erosion. Erosion by running water 



Aii Illinois field, showing the effects of water erosion. 



not only produces gullies and ditches that interfere with 

 cultivation, but greatly reduces the fertility of hilltops and 

 slopes by general surface washing. For it is the best parts 

 of the soil that are carried away by erosion. Organic 

 matter is relatively light, and so floats off ; the finer clay 

 and silt particles which, as we have seen, will remain long 

 in suspension in water, are carried away, and the coarser 



