Souls 65 
tillable surface of the earth, the shortage of moisture is 
the chief limiting factor concerned in crop growth, while 
in parts of the humid regions an excess of water in the 
soil prevents the cultivation 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. ore 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 aclay. 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 growth of crops is one of the most 
difficult phases of farming. The practical side of this 
question is discussed more fully in Chapter X. 
P 
