ARID AGRICULTURE. 103 



that the water from the saturated gravel bed be- 

 low may be drawn up to within a few inches of 

 the surface, and not so shallow as to he unsuited 

 to cultivation or to be kept too moist from the 

 supply below. So many requirements have to be 

 met that the natural conditions for the use of 

 this method are rarely found, and the system is 

 uncommon. But where the conditions are right 

 perhaps no other practice is so advantageous and 

 cheap. The ground does not have to be so accu- 

 rately graded as in other methods the irriga- 

 tion ditches needed are only the main laterals, 

 made deep enoiigh to let the water they contain 

 into the porous bed below and little labor for 

 irrigation is required. A further advantage is 

 that level, clean culture may be practiced. 

 Xearly all the advantages of the most expensive 

 pipe sub-irrigation system are obtained, and al- 

 most without expense. Soils in which sub-soil 

 is filled with free water are not suitable for deep 

 rooted plants like alfalfa. 



SUB-IRBIGA- This is the most expensive and at the same 



time tne most economical of water of all the 

 irrigation methods. It is best, when putting in 

 a system of this kind for orchards or other very 

 valuable crops, to get the services of a compe- 

 tent irrigation engineer. The details of this 

 method are usually complicated and must be 

 worked out for each farm. 



