FARM IRRIGATION 261 



soils. Some soils are so porous that if water 

 is applied to them eight or ten inches below 

 the surface, more of it will be lost to the crop by 

 downward leaching than would be lost by evapo- 

 ration if it were applied on the surface. 



Three-inch dram tiles are, on the whole, most 

 useful for sub-irrigating. They are usually laid 

 from 5 to 24 inches below the surface and from 4 

 to 12 feet apart, according to the openness of the 

 soil. Rarely is all the soil wet economically when 

 the lines of tile are more than 6 feet apart. Usually 

 each joint is closed with cement, except one or two 

 inches on the under side. They are laid like tile 

 drains, and act as drains if the soil becomes too wet. 

 The fall should be very slight. Besides tile, gal- 

 vanised sheet-iron pipes, with an open seam, and 

 perforated iron pipes are sometimes used. On 

 many soils more water will be required for sub- 

 irrigating than for surface watering, even though 

 there is no loss by evaporation, owing to the slow- 

 ness with which it moves sidewise through the soil 

 and the rapidity with which it sinks down out of 

 reach of the roots. A very porous subsoil is un- 

 favourable for sub-irrigation. 



But there is no use in discussing the pros and 

 cons of sub-watering because the expense of the 

 method is usually prohibitive. Sub-irrigation is 

 now rarely practised except in greenhouses and in 

 a few market gardens. Running water into the 

 upper end of the main of an ordinary tile drainage 

 system has been tried and with some degree of 

 success in rare instances. It is necessary in these 

 cases that the water-table should be nearly on a 

 level with the tiles. 



Under sub-irrigation mention should be made of 

 the lands that are watered naturally beneath the 



