Relation of Xonavailable Water to Hygroscopic Coefficient 19 
ing coefficient and other physical values of soils, viz, the hygro- 
scopic coefficient as determined by Hilgard, the maximum water 
capacity as determined by Hilgard, the moisture equivalent as 
defined by Briggs and McLane, and the mechanical analysis, us- 
ing the classification of separates employed by the Bureau of 
Soils. 
They conclude, that, while there are slight differences for dif- 
ferent plants and that these differences hold with the various 
soils, the wilting coefficient is practically independent of the 
plants used and is not distinctly influenced by the age of the 
plant, by the humidity of the air, by the light intensity or by the 
moisture content of the soil during the growth of the plant ; when 
the leaves curl or drop the soil moisture is not at the wilting 
coefficient but below this and above the hygroscopic coefficient; 
the death point, however, varies with the plant used, some plants 
dying much more quickly after wilting than others. After the 
'death of the plant the soil still continues to lose water thru the 
tissues of the plant. "The plants during the drying stage act 
simply as a medium for the transfer of water and, while the rate 
of loss is reduced, the final result is the same as if the air and 
soil were in direct contact." The wilting coefficient "practically 
marks the cessation of growth and so constitutes a datum from 
which the water content available for growth in a particular soil 
may be determined when the total water content is known." 
From studies of the relation between the wilting coefficient and 
other physical properties they have deduced the following 
formulas, which show also the probable error: 
^ . moisture equivalent 
WHtmg coefficient = 1M (1±o m) 
Wilting coefficient 
hygroscopic coefficient 
0.68 (1 ± 0.018) 
Wilting coefficient = ™ Qist "re-holding capacity - 21 
„ T .^. ^ . 0.01 sands + 0.12 silt 4- 0.57 clay 
Wilting coefficient = 1 -h 0 025 ' 
As the direct determination of the wilting coefficient is more 
difficult and time-consuming than the indirect method, they con- 
clude that the former may be altogether replaced by one of the 
latter, of which they prefer the determination of the moisture 
equivalent. It will be seen from the second formula that they 
place the wilting coefficient at almost one and a half times the 
hygroscopic coefficient. 
