PART THREE IRRIGATION 



CHAPTER XVIII 

 HISTORY, EXTENT, AND PURPOSE OF IRRIGATION 



Control of Soil Moisture. Attention has been called to 

 the importance of having the soil contain the proper amount 

 of moisture to furnish the best conditions for the growth of 

 crops. Plants require that the soil contain a sufficient amount 

 of moisture, not only to dissolve the plant food, but also to 

 enable them to absorb and assimilate it. Much of the plant 

 food in the soil is made available through the action of micro-" 

 scopic organisms. The vitality of these organisms depends 

 largely upon an adequate supply of moisture. As has been 

 explained, drainage is for the purpose of relieving the soil of 

 a surplus moisture; on the other hand there may be in certain 

 localities at times and in other localities at all times a defi- 

 ciency of moisture from natural sources. Irrigation is simply 

 a process of supplying water to the soil by artificial means, 

 either to make it possible to grow crops or to increase pro- 

 duction. 



Irrigation, then, is the reverse of drainage; and although 

 this be true, it is to be noted that irrigation practice has 

 many features in common with drainage. The management 

 of water is much the same, regardless of whether it is to be 

 removed from the soil as in the case of drainage or supplied 

 to the soil as in the case of irrigation. 



The importance of irrigation may be made clear by calling 

 attention to the fact that many crops, like potatoes and corn, 



in 



