696 SOILS: PROPERTIES AND MANAGEMENT 



of nozzles. Several types of nozzles are employed. The 

 amount of water that can be applied is relatively small, 

 and consequently the method is used chiefly in humid 

 regions to supplement a rather high rainfall, in the growth 

 of crops of large value. It is used in the growth of truck 

 and small fruit crops near the large eastern cities. 



The advantages of the system are : 



1. The water is conveniently applied at the desired 

 point. 2. The system may be used on uneven land and 

 without preparation of the surface. 3. There is no 

 waste of land by ditches. 4. The application of the 

 water is easily controlled by valves and by the movement 

 of the pipes. 



The disadvantages of the system are : 



1. The capacity is limited. 2. The cost is high for 

 equipping and maintaining the plant, and for developing 

 the pressure requisite to suitably distribute the water from 

 the nozzles. 3. There is possibility of injury to crops 

 where water is applied on warm, bright days, since the 

 water comes into contact with the foliage. 



600. Sub-irrigation. Sub-irrigation is the distribution 

 of water from underground pipes. These are buried in 

 the soil and perforated in such a way that the water finds 

 an outlet and is distributed by the capillarity of the soil 

 and by natural gravity flow. In greenhouses and where 

 shallow-rooted annuals are grown, lines of drain tile are 

 employed, the water flowing out at the joints. Con- 

 tinuous pipes having an open seam or perforations have 

 been used. Another method employs a porous cement 

 plug which rises a little above the supply pipe. The 

 object of the last-named method is to avoid the common 

 difficulty from the entrance of roots into the pipes. The 

 pipes must have a very slight grade in order to insure a 



