THE SOIL. 57 



to support the weight of the water raised, and so when this 

 reaches the full value of the force, equilibrium is attained. 



This upward movement of water in the soil is generally 

 described as due to "capillarity" and in this sense the 

 word is used when discussing soil physics. When the 

 water can rise from the water table to the surface, and 

 we get evaporation there, we shall have a constant stream 

 < >f water through the soil from below. In general the smaller 

 the particles of soil the higher can the water rise by capil- 

 larity but the more slowly. The particles being small the 

 interstices are small and more resistance is offered to the 

 flow of the water. 



The power of the soil to retain water, its water capacity, 

 is the result of surface tension. Capillarity, raising water 

 from a high water table or from infiltration water, is a 

 common cause of soil wetness and saltness. In a loose 

 friable soil, where the soil particles are widely separated, 

 capillarity has only a very limited action whereas in 

 firmly compressed soils the soil pores are fine, the particles 

 are close together and w r ater can be raised to a considerable 

 height. Hence, soil is rolled to increase its water capacity 

 and capillarity ; it is "fassed" to hinder the action of capil- 

 larity and prevent water being raised quite to the surface. 



The rate of evaporation from the surface of the soil 

 depends on the dryness of the air, the amount of water 

 in the soil, and the rate at which capillarity can replace, 

 from below, the water removed from the surface. Evapo- 

 ration dries and cools the soil and the extraction of water 

 causes the soil to shrink. Evaporation causes the salts 

 in the soil water to collect on the surface of the soil and 



