IRRIGATION BY SPRAYS 233 



sections. It has been applied in the growth of Sumatra 

 wrapper tobacco in Florida, and of truck crops near 

 New York, Boston and other large cities. It is therefore 

 used as a very limited supplement to the regular rainfall. 

 It is accomplished by the use of a very thorough piping 

 system with spray nozzles at sufficiently frequent inter- 

 vals to cover the area. These are connected with a rela- 

 tively large pressure-head of water at least five pounds 

 is necessary. 



The advantages of the system: (1) Economy in the 

 direct application of water to shallow rooted crops. 



(2) Convenience in applying water at the desired point. 



(3) Absence of injury from erosion or puddling the soil. 



(4) No land wasted in irrigation ditches. (5) Natural 

 climatic conditions developed by such irrigation. 



The disadvantages of the system are great: (1) The 

 large initial cost of the plant. (2) The high operating 

 expenses ordinarily necessitated to develop the pressure 

 necessary to distribute water from the nozzles, and to 

 maintain the system. (3) The limited capacity of the 

 system. (4) The large evaporation from the spray in 

 the atmosphere, and from the soil and surface of the 

 plants. 



The spray system is practicable only with special 

 crops under peculiar conditions. 



106. Sub-irrigation. Sub-irrigation often occurs 

 naturally. It is the application of water beneath the 

 surface of the soil. The structure of the land is such 

 that on many low benches and in river bottoms the 

 percolation of water through the soil and fissures of 

 the rock brings it near the surface at these lower levels, 



