196 



SOIL PHYSICS AND MANAGEMENT 



a part of the hygroscopic moisture is driven off. Condensation will 

 again take place when the temperature becomes lower. If, however, 

 the air is saturated with moisture an increase in temperature 

 increases the amount of moisture adsorbed. 



Hilgard has found that a fine sandy soil that adsorbed two per 

 cent of moisture from a saturated atmosphere at 15 degrees C. 

 adsorbed four per cent when the temperature was raised to .34 

 degrees C. A heavier soil which adsorbed seven per cent in a satu- 

 rated atmosphere at 15 degrees C. adsorbed nine per cent at 34 

 degrees C. 



(d) Organic Matter has a high adsorptive power for water, 

 especially in the form of colloidal humus. All soils contain this in 

 small amounts, at least, while some have several per cent, which 

 gives them a high hygroscopic capacity. 



(e) Humidity. The changes in humidity of the air cause a 

 variation of hygroscopic moisture at the same temperature. If a 

 soil adsorbs one unit of moisture in a saturated atmosphere at a 

 certain temperature, it will adsorb three-fourths 4 of a unit when 

 the air is three-fourths saturated, and one-half unit at one-half 

 saturation, but at one-fourth saturation will adsorb slightly more 

 than this proportion. 



Dobeneck has shown the effect of various relative humidities 

 on the hygroscopic content of quartz and humus after an exposure 

 of 24 hours at 20 degrees C. 



Percentage of Hygroscopic Moisture (Dobeneck) 5 



The Determination of the Hygroscopic Coefficient of Soils. 

 The hygroscopic coefficient of a ,soil is the amount of moisture it 

 will adsorb when exposed to a saturated atmosphere for a definite 

 time at a constant temperature. 



The determination of the hygroscopic capacity or hygroscopic 

 coefficient of soils is of considerable importance, since it gives a 

 constant for the soil that depends upon the internal surface, thus 

 giving a means of comparison. One of the best methods for its 

 determination is that of Briggs, which is to place the soil in a satu- 

 rated atmosphere at 75 degrees F., approximately 24 degrees C., and 

 let it remain until no further increase in weight is shown. It is 



