Sails 65 



tillable surface of the earth, the shortage of moisture is 

 the chief limiting factor concerned in crop g^o^\'th, while 

 in parts of the humid regions an excess of water in the 

 soil prevents the culti\-ation of vast areas of otherwise 

 fertile land. It is apparent, therefore, that soil moisture 

 is worthy of the most careful consideration. 



The quantity of moisture in the soil is not so stable as 

 the mineral constituents, but it varies from season to 

 season and from day to day. More is being added from 

 time to time, and losses occur through a number of chan- 

 nels. Even if for a short period no water is added or lost, 

 a constant movement is going on with a tendency to es- 

 tablish an equilibrium which is seldom or never reached. 

 Many forces are at work, making it difficult to determine 

 all the laws by which soil moisture is influenced. The 

 conditions of the moisture depend largely on the quan- 

 tity present and the nature of the soil, which is able to 

 hold only about a certain amount of moisture. When 

 more is added, it percolates rapidly. As the quantity 

 decreases, the tenacity with which it is held increases. 

 A sandy soil reaches the point of saturation with much less 

 water than does a clay. The condition of the moisture, 

 therefore, is not always the same with a given percentage, 

 but varies with the texture of the soil. The water of 

 the soil is usually divided into three classes, determined 

 by the percentage present. These are: (1) free, or 

 gravitational, (2) capillary, or film, and (3) hygroscopic 

 water. The maintenance in the soil of the proper moisture 

 content for the best gro\si;h of crops is one of the most 

 diflBcult phases of farming. The practical side of this 

 question is discussed more fully in Chapter X. 



