114 AGRICULTURAL ENGINEERING 



causing a thin sheet of water to flow over it, and the same 

 process may have a cooling effect in summer. This kind of 

 irrigation is practiced in Italy where a supply of warm water 

 is obtainable. 



To Kill Weeds. In rice fields the surface of the ground 

 is flooded in some instances largely for the purpose of killing 

 weeds, thus reducing the labor of cultivation. Such a 

 system also protects the crop from the ravages of birds and 

 insects. 



To Supply Fertility. Irrigation may be practiced in some 

 localities in order to supply additional fertility to the soil. 

 Some irrigation water carries a large amount of sediment 

 which is very rich in plant food. The water may also con- 

 tain soluble plant food, as phosphoric acid, potash, and 

 nitrogen. The fertility of the land along the Nile, in Egypt, 

 which has been irrigated for ages, is maintained largely by 

 the addition of fertility through the irrigation waters. 



It is true that some water supplies cannot be used for 

 irrigation, because they contain poisons injurious to plants. 

 This is often true of the water of rivers into which the refuse 

 from smelters and certain kinds of factories is discharged. 



Disposal of Sewage. In many instances the disposal 

 of sewage waters from cities has not only been facilitated, 

 but also made a matter of profit, through irrigation. Sewage 

 water, when applied to the soil is quickly purified and made 

 harmless. Sewage water is usually very rich in plant food. 



QUESTIONS 



1. Define irrigation. 



2. Why is an adequate supply of moisture in the soil important? 



3. How long has irrigation been practiced? 



4. How much land in the world is now irrigated? In U. S.? 



5. What is the main purpose of irrigation? 



6. Name and describe four other purposes of irrigation. 



