38 WILTING COEFFICIENT FOR DIFFERENT PLANTS. 



Looking at the results given in Table XII as a whole, it is seen 

 that the relative wilting coefficients of the different species range 

 from 0.92 for Japan rice to 1.13 for Colocasia. 1 The Colocasia is 

 an extremely coarse-rooted plant, and the fine fibrous roots that 

 are so characteristic of the root systems of many of the grasses are 

 here wholly absent. The result is that the Colocasia fails to 

 reduce the water content of the soil uniformly. From those parts of 

 the soil mass which are located some distance from the roots 

 water can be obtained only by capillary action, and this adjustment 

 takes place very slowly in the soil when the water content is approach- 

 ing the wilting coefficient. The determination of the moisture con- 

 tent of the whole soil mass at the time of wilting consequently gives 

 too high a value for the wilting coefficient. If the Colocasia is 

 excluded on account of its imperfect root distribution, the upper 

 Hmit of the relative wilting coefficient drops to 1.06. 



Imperfect root distribution is again encountered, to a less degree, 

 in corn, certain of the legumes, and some of the coarse-rooted plants 

 of the Great Plains. Coarse-root systems as a rule are accompanied 

 by high values for the wilting coefficient, and such values are to be 

 attributed to imperfect root distribution rather than to any inherent 

 inability of the plant to reduce the soil moisture content to the 

 point reached by other plants. 



The results of the determinations shown in Table XII are sum- 

 marized at the end of the table with regard to crops without refer- 

 ence to varieties. Reference to the mean ratios shows the slight 

 difference that exists among the various crops with respect to their 

 ability to reduce the soil-moisture content before wilting occurs. 

 Sorghum, millet, wheat, oats, barley, and the grasses are practically 

 the same. Eye and rice appear to be a little lower, while corn and 

 the legumes are slightly higher. 



In a soil whose mean wilting coefficient for all plants is 10 per cent, 

 corn, according to the above figures, would wilt with a soil-moisture 

 content of 10.3 per cent, while sorghum would wilt with a moisture 

 content of 9.8. Assuming that the roots were distributed to a depth 

 of 4 feet, this would correspond to an additional amount of water for 

 the sorghum plants in the 4 feet of soil equivalent to a rain of 0.3 of 

 an inch, all of which penetrated to the root zone. It is possible 

 that conditions might arise whereby the sorghum would be able to 

 profit from this slight additional supply of moisture, but such a 

 limited reserve would scarcely suffice to ameliorate conditions during 

 a drought to any greater extent than would a single cool, cloudy day. 



1 This statement applies to all varieties upon which six or more determinations were made. The mean 

 of a less number of determinations than sis can only be considered as indicating whether the wilting 

 coefficient for that plant is above or below normal. Ten or more determinations are essential for a quanti- 

 tative expression to which weight can be attached. 

 230 



