196 NATURE AND PROPERTIES OF SOILS 



more or less accurate for plants growing under humid condi- 

 tions. If such is the case, it can be accounted for only by the 

 fact that the soil forces in their effect on the wilting point 

 are so powerful as to over-ride any distinguishing character- 

 istics that the plant itself may possess, or at least reduce such 

 influences within the error of actual experimentation. Crops 

 wilt because they cannot get water fast enough, the wilting 

 coefficient in a humid climate being the same for most plants 

 growing on the same soil. 



Briggs and Shantz, 1 in their investigations, devised a very 

 satisfactory method for making determinations of the wilting 

 point. Glass tumblers holding about 250 cubic centimeters 

 of soil in an optimum condition were used. The seeds were 

 placed in this soil after which soft paraffin was poured over 

 the surface in order to stop evaporation, thus removing this 

 disturbing factor in the capillary equilibrium of the moisture. 

 The seedlings on germination were able to push through this 

 paraffin. While the plants were developing, the tumblers 

 were kept standing in a constant-temperature vat of water 

 in order to prevent condensation of moisture on the inside 

 of the glass. The vegetative room was under temperature 

 control. When definite wilting occurred, as determined in 

 a saturated atmosphere, a moisture determination was made 

 on the soil. The resulting figure, expressed as percentage 

 of moisture based on dry soil, represents the wilting coefficient 

 for the soil used. 2 



It is evident that the wilting coefficient will be influenced 



1 Briggs, L. J., and Schantz, H. L., The Wilting Coefficient for Differ- 

 ent Plants and its Indirect Determination; U. S. Dept. Agr., Bur. Plant 

 Ind., Bui. 230, pp. 26-33, 1912. 



2 Bouyoucos classifies the capillary water into two groups, Free (the 

 more active), and Capillary-absorbed (inner capillary). The distinction 

 is made on the basis of his dilatometer (see foot-note, page 155) results, 

 the portion which freezes at about 0°C being considered the more 

 active. The point so established by his dilatometer gives in a general 

 way the wilting coefficient as defined by Briggs and Shantz. 



Bouyoucos, G. J., A New Classification of the Soil Moisture; Soil Sci., 

 Vol. XI, No. 1, pp. 33-47, Jan., 1921. 



