Sub-irrigation 387 



The other extreme from overhead watering is sub-irri- 

 gation. This method has long been employed in green- 

 houses. The water is there conducted underneath the 

 soil in drain-tiles, and it distributes itself from the unce- 

 mented joints. The bed or bench is provided with a hard- 

 pan in the form of cement, so that the water does not leach 

 away. Earely do comparable conditions occur in nature; 

 yet sometimes in reclaimed bogs and swamps a hardpan 

 lies a foot or two beneath the surface and tiles may be 

 laid on it and receive water from the higher end. These 

 tiles may also serve the ordinary purposes of drainage. 

 Sometimes the outlets of ditches and drains in mucklands 

 are closed in dry weather and the water is held or even 

 backed up, affording a kind of irrigation. 



