LAND AND OTHER AGENTS OF PRODUCTION 175 



irregular manner. Hillside erosion is not a simple process, for in it 

 are involved the relation of the velocity of moving water to the slope 

 of the soil, the amount of organic matter incorporated in the soil, the 

 vegetal covering, the mechanical composition of the soil, and the rate 

 at which water is supplied to the surface. In addition to the surface 

 conditions of the soil, the character of the subsoil has a profound effect 

 upon the tendency to erode. The fact that a soil is or is not covered 

 with forest or grass, or contains much organic matter, or is clayey or 

 sandy, influences the rate at which it absorbs water and the amount 

 of erosion caused by the surface run-off of the water. 



The erosion of the soil occurs mainly in two ways, which are 

 markedly different: (i) as sheet erosion and (2) as the gully type of 

 erosion. In sheet erosion the water falling on the surface of the soil 

 carries off with it a small amount of soil material from every part of 

 the field. In advanced stages there appear incipient gullies, parallel 

 to each other, known as shoestring gullies. This type of erosion is 

 not so destructive of the field on which it occurs as the gully type, for 

 the removal is more uniform, and if a field is continually cultivated 

 the physical evidence of erosion may be slight. The second type of 

 erosion, or the gullying, develops where, owing to the occurrence of 

 natural depressions, the water runs off in the form of streams. These 

 cut into the soil and soon develop gulches of great depth with nearly 

 vertical sides, which grow in length, breadth, and depth with every 

 rain. This type of erosion is the most difficult to check, and renders 

 the land on which it occurs practically valueless. 



Excessive erosion results in a change in the physical condition of 

 the soil. The bodily removal of soil particles takes place from the 

 surface; there is a sorting of the soil particles, the larger and heavier 

 being deposited first and the smallest last. The result is an impaired 

 physical condition of the soil wherever this sorting action is taking 

 place. Soils composed almost entirely of either sand or clay particles 

 are not so good as those with a fair amount of each. 



The quality of the soil is greatly impaired by the continual process 

 of erosion. Rapid leaching takes place, removing a large part of the 

 soluble salts; the surface soil is often washed down to the lowlands 

 and sometimes out to the sea; gullying so defaces the land that it 

 becomes difficult to cultivate. The organic matter is one of the first 

 losses of eroded soils. Abandonment of the field follows, because the 

 land is considered too poor for agricultural use, having lost its pro- 

 ductiveness through the process of erosion. 



