WATER 107 



of a mere fraction of eroded soil. Erosion from increased 

 run'off caused by man accounts for the removal of 3 billion 

 tons of earth a year. Most of this 3 billion tons is the rich 

 topsoil that feeds plants. To see the point clearly, measure 

 erosion in terms of plant foods in the soil rather than in terms 

 of the soil itself. It is estimated that the plant food loss by 

 avoidable erosion amounts to 126 million pounds a year. This 

 is more than twenty 'one times as much as is taken out by crops. 

 To farmers this means an annual loss of at least 4 billion 

 dollars. 5 



When the topsoil is gone, the fertility of the soil is gone 

 because the underlying soil does not contain the chemicals and 

 humus that make plants grow. The worst part of this is that 

 under natural conditions topsoil is replaced at the rate of about 

 one inch every 10,000 years. 6 It will take several times that 

 many years to build up enough topsoil to make productive 

 again land eroded to the sub'soil. This means that when the 

 soil is gone, so far as we are concerned, it is gone forever. 



THE EFFECT OF UNEVEN STREAM FLOW 



Once silt, the product of erosion, gets into the bed of a 

 stream, the occupants of a river valley should know that there 

 is trouble brewing in the headwaters. The forests that pro' 

 tected the slopes of the mountains from which the river flows 

 may have been cut. Perhaps the grass meadows which absorbed 

 water for the underground source of the stream have been 

 plowed up and lie fallow. In any case, this silt is a danger sig' 

 nal that run'off may be increased during the periods of high 

 precipitation, or that there may be a scanty supply of water in 

 the dry periods when water is needed most. The reason for 

 this is that rain has run off the surface as soon as it has fallen, 

 instead of sinking into the ground. 



5 Saving Our Soil, Public Affairs Pamphlets, No. 14, Public Affairs Com' 

 mittee, Incorporated, 1937, p. 2. 



6 Letter from Soil Conservation Service. 



