FARM IRRIGATION 265 



nearly twice this amount is necessary, the average 

 for southern California being about 12 inches, 

 in addition to a rainfall of 10 to 20 inches. 



The Character o] the Soil. When water is first 

 turned upon virgin land it takes a large amount to 

 thoroughly wet the soil, especially to saturate the 

 subsoil. Newell states that on some arid soils 10 

 feet of water is often needed the first year and 

 5 feet or more per year for two or three years there- 

 after. As the subsoil becomes saturated and the 

 water-table raised, less and less water is needed, 

 until 8 to 18 inches per year or less may be 

 sufficient. 



The amount of water needed for a single irri- 

 gation varies from 2 to 4| inches, according to the 

 openness of the soil and the crop. If the soil is very 

 dry, however, as on virgin land, two or three 

 times this amount may be needed to thoroughly 

 saturate the soil to a depth of 4 or 5 feet. If the soil 

 is clayey and cracks badly, smaller and more fre- 

 quent irrigations are better, since less water is lost 

 by leaching through the cracks. 



The Kind of Crop. The deeper the roots of the 

 crop feed the more liberal may be the irrigation. 

 What is desired is to store as much water as possible 

 in that part of the soil which is laid under tribute 

 by the plants. The deeper a crop feeds the higher 

 is the * duty" of water, or the area that it ought to 

 irrigate, because less of it is lost by leaching, as it 

 is when applied to shallow-rooted crops. In arid 

 regions plants commonly root deeper than in humid 

 regions, because the subsoil is likely to be almost 

 as congenial for root growth as the surface soil. 

 Tree fruits of all kinds are deep rooted, also 

 alfalfa; the irrigations of these plants are usually 

 more liberal, but less frequent, man the irrigations 



STATE NORB1AL SCHOOL, 



tlOS AJMOBUES, CHIt. 



