120 



ELEMENTS OF AGRICULTURE 



water to dry soils. The water is usually added to the 

 soil by means of open ditches. One main channel may 

 supply a number of smaller ditches intersecting the 

 field to be irrigated, and which need be only a few 

 inches deep — little more than an ordinary furrow. 

 These irrigation ditches are filled with water from, 

 some tank or pond, much of which soaks into the soil. 

 It is not necessary that these ditches should be kept 

 filled with water, for they may be filled from time to 

 time, and the water allowed to soak out. This prac- 

 tice is called surface irrigation to distinguish it from 

 sub-irrigation, by means of which the water is added 

 to the subsoil. Underdrains, usually tile, are put in 

 with the slope running in the opposite direction to ordi- 

 nary drains, and they are occasionally flooded with 

 water, which soaks through the joints into the soil. 

 This is the most perfect system of irrigation, because 

 there is little or no loss of water through surface 

 evaporation. Unfortunately, it is much more costly 

 than surface irrigation, and for this reason but little 

 practiced except in gardens or hothouses. 



The flooding of the Nile Valley is an illustration of 

 natural irrigation. This great river annually rises far 

 beyond its banks, flooding a great stretch of country; 

 the mud it brings with it enriches the soil, and much 

 water is at the same time added. The river bottoms 

 are thoroughly irrigated once a year, and are noted for 

 their fertility. There are in our own county great 

 areas of 'soil that have been robbed of their moisture. 

 The rainfall is too uncertain to make good the loss, 

 and it is only by irrigation that most of the soil may 



