208 Bulletin 88 



Plant growth is dependent in large measure upon the presence of 

 an abundant supply of soil moisture. Surely the irrigating water 

 which actually passes thru the plant in this way does a most useful 

 service ; all that portion of the irrigating water which does not pass 

 thru the plant is wasted, so far as crop production is concerned. 



The transpiration rate, even for the same plants, varies greatly 

 according to climatic conditions, being least in humid countries and 

 increasing greatly with aridity. The rate must be high in Arizona. 

 Like the evaporation rate, it depends upon the temperature, the 

 wind movement, and the relative humidity. The characteristic of 

 high transpiration rate in this State must be acknowledged. 



Many investigators have measured the quantity of water trans- 

 pired by various plants. The U. S. Department of Agriculture* 

 conducted extensive tests of this kind in northeastern Colorado. 

 Some of their results are given in the following list, in which the 

 transpiration is stated as the number of pounds of water required to 

 produce a pound of dry matter, sometimes called the transpiration 

 ratio. The soil used was "rich, dark loam." 



WATER ABSORBED BY PLANT ROOTS DURING GROWTH 



(Based on total dry matter produced) 



Crop 



Alfalfa 



Barley, average of 4 varieties... 

 Wheat, average of 5 varieties . . . 



Potato 



Corn, average of 3 varieties 



Sorghum, average of 5 varieties. 



Pound.s of water per 

 pound of dry matter 



1068 

 539 

 507 

 448 

 369 

 306 



When the production of grain alone was considered, the water 

 requirement of wheat was found to be 1357 pounds of water per 

 pound of dry matter, and of sorghum 790 pounds of water. The 

 sorghum family of plants is specially adapted to arid climates and 

 in particular to those localities where the limitations of w^ater 

 supply are felt seriously. It is evident, too, that alfalfa is the 

 water gourmand, suggesting therefore that farmers, especially those 

 under pumping plants wdth high lifts, should restrict their alfalfa 

 to the amount needed for feeding their own necessary stock. 



Most investigators believe that the texture and tilth of the soil, 

 and the fertility, have a pronounced effect on the transpiration ratio. 

 Thus, on clayey soils and very sandy soils plants transpire more 

 water per unit of crop produced than on good loam ; and compara- 



*U. S. Dept. Agri., Bureau of Plant Industry, Bull. No. 284, 1913. 



