420 IRRIGATION FARMING. 



the supply received or contained in the underlying 

 strata, and the chara<5ler of the soil operated upon. 

 Two other points closely allied to these are the storage 

 capacity underneath and the manner of cultivation. 



Farmers should make themselves most thoroughly 

 acquainted with the subsoil on their estates down to a 

 depth of at least four, but preferably six or eight feet. 

 Similarly, no irrigator should be ignorant of the time 

 or amount of water required to wet the soil to a given 

 depth. A definite knowledge of the rapidity with 

 which irrigation water penetrates downward and later- 

 ally in the soil should form a part of the mental equip- 

 ment of every irrigator, particularly in arranging for 

 subirrigation. Supposing the moisture to have 

 reached the depths of the soil, whether from rains or 

 from irrigation, it is essential that proper means be em- 

 ployed for retaining it in the land and especially to 

 prevent evaporation. As has been set forth quite 

 elaborately in this work in the chapter on soils, this is 

 best accomplished by a dust mulch on the surface of 

 loose, well-tilled soil. Where this principle is well 

 understood, it is considered that a surface layer of 

 three inches or so in thickness is sufficient for effec5tive 

 protec5lion, and this rule applies in subirrigation quite 

 as materially as when water is applied by the various 

 surface methods. 



The difference between water applied to the surface 

 by irrigation and that applied below the surface eigh- 

 teen inches to two feet, is that in the former case there 

 is much evaporation after the water is applied, and the 

 air has not free access to the soil and roots of the plants 

 for a day or two. In the latter the subsoil is saturated 



