188 NATURE AND PROPERTIES OF SOILS 



from a cropped soil, finding the loss due to transpiration 

 alone. 



An objection to tlie 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 en- 

 ters, since the evaporation from the bare soil is not the same 

 as that from a soil covered by vegetation because of the effect 

 of shading. Moreover, due to the action of the roots, less 

 water is likely 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 beeau.se of the great num- 

 ber of factors that under normal conditions may vary the 

 transpiration ratio. The following data drawn from various 

 investigators working by the general methods^ already out- 

 lined, give some idea of the water transpired by different 

 crops, due allowance being made for various disturbing fac- 

 tors. (See Table XXXIV, page 189.) 



100. Factors affecting transpirlation.^ — It is obvious 

 from the figures quoted that the transpiration ratio of a crop 

 is the resultant of a number of influences.^ The factors may 

 be listed under three heads, as follows : 



1. Crop. — Difference due to different crops and to vari- 

 ations of the same crop. 



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



Montgomery, E. G., Methods of Determining the Water Bequire- 

 ments of Crops; Proc. Amer. Soc. Agron., Vol. 3, pp. 261-283, 1911. 



Also Briggs, L. J., and Schantz, H. L., The TVater Beqmrement of 

 Plants; V. S. Dept. Agr., Bur. Plant Ind., Bui. 285, 1913. 



* Kiesselbach, T. A., Transpiration as a Factor %n Crop Production; 

 Nebr, Agr. Exp. Sta., Res. Bui. 6, June, 1916. 



®A complete review of the literature concerning the climatic and 

 soil factors in their effect on transpiration may be found as follows: 

 Briggs, L. J., and Shantz, H. L., The Water Requirement of Plants; 

 U. S. Dept, Agr., Bur. Plant Ind., Bui. 285, 1913. 



See also, Briggs, L. J., and Shantz, H. L., Daily Transpiration dur- 

 ing the Normal Growth Period and its Correlation with the Weather; 

 Jour. Agr, Ees., Yol. VII, No. 4, pp. 155, 212, Oct., 1916. 



