THE FORMS OF SOIL WATER 



205 



moisture was determined by exposing the air-dry soil at 

 15° C. to a saturated atmosphere and then drying at 200° 

 C, illustrate this point : — 



Hygroscopic Capacity of Various Soils 



Soils 



Per cent Clay 

 Material Remain- 

 ing in Suspension 



after Standing 

 for 24 Hours 



Hygroscopic 



Water Expressed 



in Percentage 



15 clays . . 

 7 clay loams 

 9 loams . . 

 4 sandy loams « 

 4 sands . . 



31.97 



17.15 



12.06 



7.39 



2.93 



10.45 

 6.06 

 5.18 

 2.50 

 2.21 



Apparently, the finer the soil, the greater is the hygro- 

 scopicity. The finer the soil, the higher also is the per- 

 centage of clay, and consequently the greater is the amount 

 of material likely to be present in a colloidal state. As a 

 matter of fact, the hygroscopic moisture as shown above 

 is roughly proportional to the clay; and as clay, espe- 

 cially the finer forms, is largely colloidal in nature, the 

 colloidal content of a soil practically determines the hygro- 

 scopic content. This fact is the basis for Mitscherlich's * 

 method of colloid estimation, in which hygroscopic mois- 

 ture determined under certain controlled conditions is 

 used as a relative measure of colloidal content. The vari- 

 ous grades of particles constituting the textural make-up 

 of a soil, then, do not possess the same weight in the deter- 

 mination of hygroscopicity, the dominant grade being 

 clay, especially that part which has, by either physical 



1 Mitscherlich, E. A. This text, paragraph 111. 



