272 



SOIL PHYSICS AND MANAGEMENT 



method is specially adapted to inter-tilled crops, such as corn and 

 potatoes, and is used extensively for cereals, alfalfa, and orchards. 



(c) Sub-Irrigation. The method of sub-irrigation is prac- 

 ticed only to a very limited extent because of the great initial cost 

 making it almost prohibitive. Iron, concrete or wooden pipes may 

 be used, but digging the trenches for placing these is expensive. 

 The roots clog the openings and in time impair the usefulness of 

 the system. 



A form of natural sub-irrigation is practiced in the West where 



Fio. 124. Method of irrigating by overhead sprays. Adapted to small fruits and 

 vegetables in humid areas. (Fortier's Use of Water in Irrigation.) (Courtesy McGraw- 

 Hill Book Company.) 



the soil is sufficiently porous so that no underground pipes are 

 necessary. Former irrigation has brought the water table near 

 the surface, and now the object to be accomplished is to keep the 

 water table sufficiently near the surface so that capillary water from 

 it will supply the crops. An impervious stratum is necessary at a 

 depth of a few feet. A tract of 60.000 acres is irrigated in this way in 

 the upper Snake River Valley, Idaho. Parts of the San Luis Valley, 

 Colorado, are irrigated in the same manner. The ditches are from 

 50 to 250 feet apart. 



(d) Surface Sprinkling and Overhead Sprays. This method 

 is adapted only to small areas and is one of the most expensive as 



