METHOD OF IRRIGATION 



191 



expense. Still worse, plant roots, always in search of 

 water, are gradually directed to the openings hi the under- 

 ground pipes, and fill them so completely that the flow 

 of water is either greatly diminished, or entirely stopped. 

 For this reason every sub-irrigation system has either been 

 abandoned or has been maintained only hi spite of the 



CA+/AL 



FIG. 32. Plan of a sub-irrigated farm in Idaho. 



great cost of keeping the water outlets free from plant 

 roots. The best that can be said about sub-irrigation is 

 that the method has not yet been perfected, and that it 

 offers a fine field for the agricultural inventor. 



One kind of sub-irrigation of extremely limited appli- 

 cation has proved successful. In certain localities are 

 found somewhat sandy soils, 1 to 5 feet in depth, under- 

 laid by an almost impervious clay. Ditches are dug at 

 intervals of }/% to % mile. The water flowing through these 



