CULTrV'ATION. 191 



it of a portion of its heat, which is carried down to 

 equalize the temperature for the wliole depth. The 

 heat of tlie rain-M'ater itself is given up to the soil, 

 leaving the water from one to ten degrees cooler, 

 when it passes out of the drains, than when received 

 by the earth. 



This heating of the lower soil of course renders it 

 more favorable to vegetation. 



4. Under-draining hastens the decomposition of 

 roots and other organic matters in the soil, by ad- 

 mitting increased quantities of air, thus supplying 

 oxygen, which is as essential in decay as it is in com- 

 bustion. It also allows the resultant gases of de- 

 composition to pass aw^ay, leaving the air around 

 the decaying substances in a condition to continue 

 the process. 



This organic decay, besides its other benefits, pro- 

 duces an amount of heat perfectly perceptible to the 

 smaller roots of plants, though not so to us. 



5. Draining accelerates the disintegration of the 

 minerals in the soil, by admitting water and oxygen 

 to keep up the process. This disintegration is ne- 

 cessary to fertility, because the I'oots of plants can 

 feed only on matters dissolved from surfaces j and 

 the more finely we pulverize the soil, the more sur- 

 face we expose. For instance, the interior of a stone 

 can furnish no food for plants ; wliile, if it were 

 finely crushed, it might make a fertile soil. 



Anything tending to open the soil to the air facili- 

 tates the disintegration of its particles, and thereby 

 iuereases its fertility. 



