238 THE FIRST BOOK OF FARMING 



Influence on soil ventilation. 



Lowering the free water allows a deeper pene- 

 tration of air and, therefore, a deeper root develop- 

 ment and enables crops to better resist dry periods. 



Influence on soil temperature. 



Lowering the free water in the soil influences soil 

 temperature : 



By diminishing the amount of water to be heated. 



By checking evaporation. 



By letting warm showers sink down into the soil. 



By increasing ventilation and therefore permit- 

 ting the circulation of warm air in the soil. 



The cropping season is lengthened by causing the 

 soil to be warmer and drier earlier in the spring and 

 later in the fall. 



Influence on plant food in the soil. 



Covered or under drains check losses of plant 

 food that occur with surface and open ditch drains. 

 They render available more plant food, for lowering 

 free water and increasing ventilation : 



Deepen the feeding area of the roots. 



Aid the process of nitrification. 



Aid chemical changes which make plant food 

 available. 



Check denitrification. 



LOCATION OF DRAINS 



As gravity is the force that is to take the surplus 

 water from the soil, the outlet of the drainage sys- 

 tem should be at the lowest part of the area to be 

 drained. 



