FERTILITY AND HOW TO MAINTAIN 47 



time the supply will be so nearly exhausted as to necessitate the return of 

 this element to the soil in some commercial form. In some soils it is 

 already necessary for most profitable crop production. 



Loss by Erosion. The loss of soil fertility by erosion is more serious 

 than the loss by plant removal. In this way there is not only a loss of 

 plant food but a loss of a portion of the soil body itself. The millions of 

 tons of finest soil particles and organic matter carried annually to the 

 ocean by the rivers of the United States are a monument to careless soil 

 management. This waste may be witnessed everywhere. The removal 

 of the most fertile part of the soil is not only a loss to the soil, but is often 

 a menace to navigable streams which are filled up with this material. An 

 enormous expenditure on the part of our national government is necessary 

 in dredging them out and making them again navigable. This erosion 

 also becomes a menace to our great city water supplies, necessitating ex- 

 pensive filter plants to remove the suspended matter and purify the water. 

 It also frequently does damage to other land subject to overflow, and on 

 which the deposits may be left. 



The great problem, therefore, seems to be the control of the rain that 

 falls upon the land. A portion of this may pass over the surface, carrying 

 with it small amounts of the surface, which in the course of time has been 

 largely exhausted of plant-food elements. This loss should be accom- 

 panied by a renewal of the soil from below. The addition of new soil 

 below should keep pace with the removal from the surface if permanent 

 soil fertility is to be maintained. The remainder of the rainfall should 

 find its way into the soil. A portion of this may pass off into the drainage 

 waters, removing certain soluble material that without such drainage 

 might accumulate in the course of centuries to the detriment of plant 

 growth. Another portion should return to the surface, bringing with it 

 the soluble constituents of the soil and leaving them near the surface for 

 the use of growing plants. 



Preventing Soil Erosion. Water escaping from the soil by means of 

 underdrainage never carries with it any of the soil material other than 

 the slight portions that are soluble. It is, therefore, essential to establish 

 systems of farming that will enable a large proportion of the rainfall to 

 penetrate the soil; and to remove the excess of water by underdrainage 

 when nature fails to provide such a system. Erosion may be largely pre- 

 vented on most farms by deep plowing and by keeping the soil covered 

 as much as possible with growing crops or their remains. Deep plowing 

 encourages an increased penetration of the rainfall and, therefore, reduces 

 the amount passing over the surface of the soil. The presence of growing 

 plants retards the movement of surface water and holds back the soil 

 particles. An abundance of roots in the soil helps to hold it together and 

 prevent erosion. The application of barnyard and green manures 

 also retards erosion. In some places terracing the soil to prevent 

 erosion becomes necessary, but it is a costly and cumbersome method 



