420 Irrigation and Drainage 



the lime which in its turn tends to force the fine 

 clay particles into larger compound clusters, thus ren- 

 dering the soil more open, and hence better drained, 

 better ventilated, and at the same time better and 

 more thoroughly occupied by the roots of plants. 



But all of these changes, which result directly 

 from lowering the ground -water surface, are only 

 means which make underdrainage more effective in 

 ventilating the soil. In an underdrained field, where 

 lines of tile are laid 3 to 4 feet deep and 50 to 100 

 feet apart, there is provided a very effective system 

 of soil ventilation as well as of drainage ; for with 

 every fall of the barometer and rise of soil tempera- 

 ture, some of the deeper soil -air expands and drains 

 away through the lines of tile. Then, when the 

 barometer rises again, or when the soil temperature 

 falls, a volume of air equal to that which left the 

 soil under the other conditions now enters it again, 

 not only through the surface of the ground, but 

 also through the tile drains. It is thus seen that 

 a deep, well -laid system of tile drains permits the 

 free oxygen of the air to reach the roots of plants 

 both from above and below. Under these condi- 

 tions, the roots of crops are better supplied with 

 oxygen ; nitrates develop faster and deeper in the 

 soil ; there is less occasion for denitrification to set 

 in, and so larger yields result. 



When deep underdrainage has permitted the roots 

 of plants to penetrate the soil from 3 to 4 feet and 

 there withdraw moisture, this action on their part 

 becomes a means for drawing air into the ground, 



