130 DRAINAGE AND IRRIGATION 



Effects of Drainage. When underdrains are properly installed 

 the benefits resulting from drainage are of several kinds. We 

 usually think of the removal of surplus water as the chief ad- 

 vantage; but other results are as important. The feeding area 

 for roots is deepened. Instead of the roots remaining near the 

 surface during the wet season they may extend to a much greater 

 depth at all times. This results in less damage during the hot, 

 dry weathe'r of midsummer. Underdrains cause the upward 

 movement of surplus water. This often prevents surface wash- 

 ing and will aid in drawing air into the soil. As the water 

 moves downward air must follow. In the spring the entrance of 



W,/d Gross es /,53 O, 669 



<9a/s 739, 632 



Tame Grasses one/ C/over 6O5,Q26 



Wheat 548J73 



Orch o re/ fruits on c/ Grapes 23 6, 3 85 

 - Gro/ns cutareen 209,363 



Suyor Beets /83,308 



- Potatoes /68,O/^ 

 . Corn /33,673 



/ropjco/an d sub-tropica/ Fruits 99, 43/ 



A//o//?er 330^83 



FIG. 86. Acreage of crops irrigated in 1909 total 7,241,561 acras. 



air by this means has a beneficial effect in warming the soil. Crops 

 may be started much earlier and the growth be more rapid. Such 

 conditions favor the growth of market garden crops, early potatoes 

 and fruits which are to mature in time for the best market prices. 

 A corollary to all of these effects of drainage is an increase in bac- 

 teria and chemical action in the soil and subsoil. This increase 

 is due chiefly to the admission of oxygen and nitrogen among 

 particles of soil which have not been sufficiently supplied with 

 air. Underdrains ventilate the soil by admitting oxygen and 

 nitrogen and by removing carbon dioxide. Much of the surplus 

 carbon dioxide may be dissolved in soil water which is carried 

 downward to the drains. 



