IRRIGATION EXPERIMENTS IN IDAHO. 89 
system can be devised that will evenly apply a small quantity at each 
irrigation, porous soils can be 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 being equal, 
porous soils which have a low water-holding capacity and the very 
impervious soils which absorb 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 
1to3feetindepth. 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 
