360 SOIL PHYSICS AND MANAGEMENT 



any degree of accuracy the total quantity of material moved by 

 water, but it must be many times as much as is deposited from sus- 

 pension in lakes and the seas. 



Some geologists hold that the land surface of the earth as a 

 whole is being lowered by erosion one foot in six thousand years. 

 The combined loss from surface erosion and from solution by 

 ground waters amounts to one foot in about 4100 years. 



When a stream emerging from a narrow valley spreads out over 

 the bottom land, the velocity of the water is checked and its load 

 of stones, gravel and sand is deposited on the rich alluvial land. In 

 this way much valuable soil is buried completely by material of little 

 immediate use to plants. The finer material, if unweathered and 

 deficient in organic matter, may be almost equally worthless until 

 acted upon by the regular soil-forming agencies. If the deposition 

 is rapid there is little chance for soil to form, but if deposition is 

 prevented for a time this almost worthless material becomes a valu- 

 able soil. 



1. Removal of Organic Matter and Nitrogen. The surface 

 soil contains the greater part of the organic matter, and so is the 

 richest, most productive part of the soil. The removal of any ap- 

 preciable amount of this stratum reduces the amount of plant food, 

 especially the nitrogen, rendering the soil less productive than 

 formerly. The following results from pot culture tests in the green- 

 house show their great need of nitrogen : 



Yields from Eroded Hill Lands 3 (Bushels per Acie) 



2. Changes Physical Character of Soil. The removal of the 

 surface soil exposes the yellowish or reddish subsoil, which is heavier 

 and more difficult to work than the original surface soil. These 

 exposures of subsoil are locally known as " clay points." They are 

 less productive than the original land. In some residual and glacial 

 soils with a wide range in size of particles the texture of the sur- 

 face may be changed from a fairly heavy to a sandy or gravelly 

 soil by the removal of the silt and clay, leaving only the coarse mate- 

 rial which was too large to be carried away by the water. 



