HYGROSCOPIC MOISTURE 



197 



then dried at 100 decrees ('. The difference gives the hygroscopic 

 coefficient of the soil. Hilgard has used a temperature of 200 

 degrees 0. in the determination of hygroscopic capacity, which 

 comes nearer, probably, reaching the point of absolute dryness of 

 soil. In this determination the soil should he spread out in a very 

 thin layer, so that as large a surface as possible may be exposed 

 directly to the saturated air. 



The hygroscopic coefficient of soils may he determined indirectly 

 by using other constants to which the hygroscopic coefficient bears 

 a definite relation. The- formnhv are as follows: 



(a) Hygroscopic coefficient ' = wilting coefficient X 0.68. 



(b) Hygroscopic coefficient = moisture equivalent x <>..'J7. 



(c) Hygroscopic coefficient = (moisture holding capacity - 



* X 0.234. 



(d) Hygroscopic coefficient = 0.007 sands + 0.082 silt + 0.:M) 



-j- organic matter). 



21) 



(clay 



For wilting coefficient see page 212; mx>isture equivalent, page 

 202, and moisture holding capacity, page 20!). 



The Use of Hygroscopic Moisture. It was formerly believed 

 that plants were able to use some hygroscopic moisture, but later 

 investigations seem to show that this is not possible. Permanent 

 wilting has been taken as the point at which plants cease to obtain 

 sufficient water from the* soil, and at this point, they still contain 

 some capillary moisture, although the film is quite thin. 



Determinations of the moisture content of soils at wilting have 

 been made by Briggs and Shantx, and are given in the following 

 table : 



Relation of Hygroscopic Coefficient 7 to the Willing Coefficient 



Amount of 



capillary water 



remaining 



per rent 



0.4 

 1.1 

 1.3 



Tn the work of Briggs and Sbantx. the determination of the 

 wilting coefficient of soils always shows the presence of some capil- 

 lary water. Even at the death point of plants soils show more than 

 the hygroscopic water present. 



