WATER OF SOIL IN ITS RELATION TO PLANTS 245 



transpired from the leaves. The former loss may be 

 eliminated from calculations in two ways : (1) by covering 

 the soil in some way so that evaporation is absolutely 

 checked and the only loss is by transpiration ; or (2) by 

 determining the evaporation from a bare pot and, by 

 subtracting this from the total water loss from a cropped 

 soil, rinding the loss due to transpiration alone. 



An objection to the former method is that any covering 

 which interferes with evaporation interferes with proper 

 soil aeration also and may render soil conditions abnormal. 

 In the second method, however, an even more serious 

 error enters, since the evaporation from a bare soil is 

 not the same as that from a soil covered by vegetation 

 because of the shading effects. Also, due to the action 

 of the roots, less water is likely to be allowed to move 

 to the surface by capillary attraction in the cropped soil. 

 Therefore, any data that may be quoted can be only 

 general in its application, not only because of the errors 

 of determination but also because of the great number of 

 factors that under normal conditions may vary the 

 transpiration ratio. The data on the following page, 

 drawn from various investigators working by the gen- 

 eral methods * already outlined, give some idea of the 

 water transpired by different crops, due allowance being 

 made for various disturbing factors. Below the data 

 regarding transpiration will be found the citations to 

 the work of the various authors as well as a few 

 notes regarding their experimental procedure. 



1 A brief discussion of the various methods is found as follows : 

 Montgomery, E. G. Methods of Determining the Water 

 Requirements of Crops. Proc. Amer. Soe. Agron., Vol. 3, 

 pp. 261-283. 1911. Also Briggs, L. J., and Shantz, H. L., 

 The Water Requirement of Plants. U. S. D. A., Bur. Plant 

 Ind., Bui. 285. 1913. 



