SUB-IRRIGATION 235 



Plant roots seek the most moist soil which is short of 

 saturation, and therefore they are drawn toward and 

 tend to concentrate around and in the lines of tile, 

 just as roots are found to do where drain tiles carry living 

 water through dry soil. This is the greatest disadvantage 

 of the system. Especially is this true in orchard work. 

 It is more adapted to shallow-rooted annual crops, and 

 to soils of strong rapid capillary power, such as fine 

 sand and coarse silt loam or loam soil. 



The amount of water to be added at one time must 

 be determined chiefly by the texture and structure of the 

 soil, or more specifically its water capacity, and the 

 supply of water available. Under arid conditions, it is 

 generally advisable to apply as much water as can be 

 held within the root zone by capillarity without loss 

 from percolation. Frequent small applications should 

 be avoided, because of the large proportionate loss from 

 surface evaporation. (See page 197.) Also, there is a 

 stronger tendency to the accumulation of alkali salts at 

 the surface, because of the larger evaporation. On the 

 other hand, less frequent large applications of water, 

 particularly under any but the flooding system, where 

 a crop occupies the land, permits the creation and main- 

 tenance of a mulch to conserve moisture; besides which, 

 the deep distribution of the water insures a deep distri- 

 bution of the roots, where they are not only in contact 

 with a larger moisture reservoir, but also with a larger 

 food-supply than is available to shallow-rooted plants. 

 It is a fact of common experience that in arid regions 

 crops generally root deeper than in humid regions. 



A common accompaniment of irrigation, certainly 



