NATURE OF SOIL 311 



not only produces gullies and ditches that interfere with 

 cultivation, but greatly reduces the fertility of hilltops and 

 slopes by general surface washing. For the best parts 

 of the soil are carried away by erosion. Organic mat- 

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

 silt particles, as we have seen, remain- long in sus- 

 pension in water and are carried away, while the coarser 

 portions are left behind. And it is just this organic mat- 

 ter and the finer soil particles that contain the best part 

 of the plant food. No wonder then that the hilltops have 

 a thin poor soil, and that the valleys are noted for their 

 fertility. 



Prevention of erosion. While soil erosion can not be 

 wholly prevented, it can be greatly checked. And nature 

 suggests one effective remedy in covering all soil with vege- 

 tation. It is bare soil that washes and blows away. Even 

 a steep hillside when covered with grass is reasonably free 

 from erosion. For the stems tend to delay the downward 

 rush of water, thus causing it to soak into the ground, and 

 the roots bind the soil together. This indicates that steep 

 slopes should be used for pasturage, meadows and wood 

 lots rather than for cropping. The addition of organic mat- 

 ter in the form of manure also lessens the danger from 

 washing, since it increases the capacity of the soil to absorb 

 water, and also tends to bind the soil together. 



Hillsides when tilled should be plowed along the slope 

 rather than up and down, and in many sections it is neces- 

 sary to terrace the hillsides. This delays the forming of 

 rivulets and favors absorption of water. Every furrow 

 leading down the hill is the beginning of a gully in lines of 

 heavy rains. Even the marks left by the wheels of a corn 

 planter, unless leveled over by harrowing, will serve as 



