ABSORPTION OF MOISTURE BY SOILS. 



241 



power in some kinds of soil it diminishes it in others. Thus 

 Zenger has shown that fine quartz sand absorbs about twice as 

 much water as that which is coarse ; on the other hand, fine 

 brick-clay is not so absorbent as coarse. 



647. Admixture of heterogeneous matters with soil generally 

 lowers the absorptive and retentive power both of ihe soil and of 

 the added substances. Treutler examined certain soil mixtures 

 in the following manner : fifty grams of the soil were placed in 

 one hundred cubic centimeters of water for twenty-four hours, 

 the excess of water was allowed to drip awa}-, and the amount 

 then retained noted. The following are among his results : 



From Treutler's tables it appears that the absorptive and 

 retentive capacity of a mixture of two substances may equal 

 that of the constituents, but that generally it becomes lower. 



648. A soil may be so fine and compact that rain will not 

 readily penetrate it ; or on the other hand it may be so porous 

 as to allow the water which falls on it to pass rapidly down 

 through it. A soil of proper texture will receive the rains, and, 

 as has been shown by the foregoing paragraphs, retain a certain 

 amount in its pores, the excess draining away. 



649. Evaporation of water goes on continually from the sur- 

 face of moist soil, unless the atmosphere is saturated, and the 

 amount of evaporation depends largely upon the amount of 

 moisture present in the state of vapor in the atmosphere at any 

 given time. But the retentive power spoken of above (which 

 is plainly opposed to evaporation) is very different in different 

 soils ; for this reason about three times as much water evaporates 

 from quartz sand as from the same amount of humus equally 

 exposed for a given time. When by evaporation the soil be- 

 comes dry at the surface, a draft is made upon the supply of 

 water retained in it at a greater depth, and this water then rises 

 by capillarity to the drier layers. It is therefore said that there 

 is a constant movement of water in the soil. 



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