Soils 



45 



Sub-irrigation. The distributing of water through pipes 

 or lines of tile placed underneath the surface of the ground 

 is called sub-irrigation. Perforations in the pipe or joints be- 

 tween the tile allow the water to pass into the soil. It is carried 

 to the roots by capillary attraction. This method is used in 

 humid climates and is applicable to shallow-rooted crops. One 

 difficulty is that the pipes are likely to become clogged by roots. 



FIG. 17. Vegetables grown under irrigation. 



When a soil is sandy and is underlaid at a depth of three or four 

 feet by a stratum of clay, conditions are favorable for the in- 

 stallation of a sub-irrigation system, because the soil can be 

 saturated and the pipes emptied quickly. Since the pipes 

 are usually empty, the roots do not enter them as they would 

 if they held water for a greater part of the time. This system 

 is used largely in truck-farming. In the vicinity of Sanford, 

 Florida, it has been extensively employed for irrigating soil 

 used for celery and other vegetable crops. 



