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 op Various Soils 



Soils 



15 clays . . 

 7 clay loams 

 9 loams . . 

 4 sandy loams 

 4 sands . . 



Per cent Cla.y 

 M \TrEi vL, Rbmain- 

 ixG IN Suspension 



iB^TFR Standing 

 FOB 24 Hours 



Hygroscopic 



Water ExPREbsED 



IN PERCE^TAGES 



10.45 

 6.06 

 5.18 

 2.50 



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 show^n 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 Mitscherlieh, E. A. This text, paragraph 111. 



