THE SOIL AND SOIL FORMATION 8l 



feet and to satisfy entirely the capillary force of a 



dry soil. 



In case sufficient rain falls to penetrate to only a few 

 inches, the moist surface soil soon begins to give up its 

 moisture in two directions. The moister particles give 

 up their water to the drier soil particles beneath, as was 

 described above, and as soon as the sun warms and dries 

 the air at the surface, moisture is there evaporated, and 

 there is a consequent movement of 

 water within the moist upper zone or 

 layer of soil toward the sun-dried sur- 

 face particles. These movements will 

 both continue for a time, but soon the 

 zone of moist soil, will have given up 

 Sufficient water so that it is no moister 

 than the subsoil below, and the down- Sbe oug water the i S 



j . ,. T- run into the center of 



ward movement will cease. But since the mass of son. The 



.. 1-1 , 1 ., water is carried away 



the sun and wind are m almost daily gffi ^"ffi*^^ 



action in summer in evaporating water HVavity ce cau f ses a a iU si r ight 



from the surface of the soil, there is a SkTardXTin SuS 



Bcap the" 



movement of capillary moisture upward ?X Uon bW B much c 



nearly all the time. Part of this mov- SSS 



e . . , ditions, causes the water 



ing mass 01 water is intercepted, in its to go upward andside- 



_ i wise as well as down- 



slow upward now, by plants, which take ward - 



it into their roots, pass it to the leaves and there tran- 



spire it into the atmosphere. 



Water-holding power varies with different soils. 

 Some soils retain much more hygroscopic moisture 

 when air-dried than others, some soils will hold a much 

 larger amount of capillary moisture than others, and 

 soils vary greatly in the amount of water which can flow 

 into their interstices as ground water. Thus a hundred 

 pounds of soil containing clay or vegetable matter, when 

 drenched with water, will cling to, and prevent from 

 running out of, its body much more water than will a 

 sandy soil. In soils which are so constituted as to hold 



