IRRIGATION EXPERIMENTS IN CDAHO. 39 



system can ho devised that will evenly apply a small quant il y al each 

 irrigation, porous soils can he irrigated and made to produce successful 

 crops with no more water than is required for the irrigation -of the 

 medium or rather impervious soils. 



NUMBER OF IRRIGATIONS TO APPLY. 



The number of irrigations required during the season is dependent 

 upon the soil, climate, and class of crop. Other things heing equal, 

 porous soils which have a low water-holding capacity and the very 

 impervious soils which absorh only small quantities at each appli- 

 cation, will require irrigation oftener than soils of the medium types. 

 Alfalfa and pasture grow throughout a longer season than the grains 

 and consequently require more irrigations. The number of irriga- 

 tions required during the season may vary from 1 on winter grain in 

 the high altitudes of Montana to as many as 20 on alfalfa in southern 

 California and Arizona, if the same be planted on porous soils. The 

 number of irrigations required under conditions similar to those of 

 Idaho is approximately three for grain and five for alfalfa. The 

 average number applied in this investigation on all types of soils was 

 3.1 for all grains and 5.4 for alfalfa. Clean-cultivated orchards on 

 deep soils of medium texture will seldom require more than two or 

 three irrigations per annum. A safe rule to follow for hay and 

 pasture grasses is to apply a sufficient number of light irrigations 

 during the season so that plants will not wilt for lack of water. 



PROPER QUANTITY OF WATER TO APPLY AT EACH IRRIGATION. 



A study of the tables included in this report shows that the volumes 

 of water which have been applied to the various tracts at each irriga- 

 tion have varied widely. It is not uncommon to find soils which are 

 so impervious that they will absorb barely 0.1 to 0.15 foot in depth 

 at each irrigation, or soils so porous that they can be made to absorb 

 1 to 3 feet in depth . The investigation has made it plain that a depth 

 of 0.1 to 0.2 foot for one irrigation is insufficient if economy of water 

 is desired, for the moisture forced into the soil does not last long 

 enough, thus necessitating too many irrigations during the season. 

 An unavoidable loss from evaporation invariably occurs during and 

 immediately after each irrigation, and it is therefore desirable to 

 apply as few irrigations during the season as will be required to main- 

 tain a sufficiently high moisture content in the soil for good plant 

 growth. The investigation indicates that, generally speaking, from 

 3 to 6 acre-inches at one application is the correct quantity to apply 

 and that impervious soils should be so manipulated that they can be 

 made to absorb at least the smaller amount, while the porous soils 

 should be so handled by using large irrigation heads that they can be 



