144 IRRIGATION PRACTICE 



ous plant-growth, they also tend to diminish the quantity 

 of water required for the production of one pound of dry 

 matter. That is, so far as these factors are concerned, as 

 the plant becomes more and more thrifty the smaller 

 becomes the transpiration ratio. The more vigorous a 

 plant is, the more economically can it use the water at 

 its disposal. 



102. Varying quantities of water. Of greatest impor- 

 tance in the consideration of the economical use of water 

 by plants is the effect of varying quantities of water. 

 Under irrigation, much or little water may be applied 

 at the will of the farmer. Upon the proper manipulation 

 of this characteristic factor, irrigation agriculture will 

 stand or fall. It is, therefore, of prime importance to 

 know how the production of crops is affected when the 

 quantity of water applied is varied. 



Many experiments on this subject have been made 

 Jately, but not enough to set forth fully the principles 

 involved. Most of the leading students of water in rela- 

 tion to agriculture have lived in humid countries, where 

 the only important control of soil water is the conserva- 

 tion of the rain or snow-water in the soil upon which 

 it falls. Only in recent years has serious attention been 

 given to the subject from the direct point of view of irriga- 

 tion. Moreover, most of the experiments on this subject, 

 many of high value, have been made in pots, under con- 

 ditions not strictly comparable with the conditions of 

 practical irrigation. Usually, the soils have been main- 

 tained at definite degrees of wetness. To maintain these 

 conditions, water was added to the pots every day or 

 every few days, so that it could be said, at the end of the 

 experiment, that the soil had been kept practically at 

 that degree of saturation throughout the whole experi- 



