THE BIOLOGICAL CONDITIONS IN THE SOIL 223 



Rootlets penetrated II. and entered III. 



No similar studies of agricultural soils seem to have been 

 made. 



Soil physicists, on the other hand, have divided the soil 

 water under the following headings : — 



1 . Hygroscopic moisture, being the moisture that a dry 

 soil can absorb from a moist atmosphere. 



2. Capillary water, being water held by capillary or sur- 

 face forces which, being much stronger than gravity, prevent 

 it being lost by drainage. 



3. Gravitational water, being the excess water which is 

 capable of draining away. 



The hygroscopic moisture is marked off from the capillary 

 water by the point called the hygroscopic coefficient, the per- 

 centage of moisture in a soil which, initially dry, has been 

 placed in a saturated atmosphere and absorbed water till it is 

 in equilibrium therewith (Hilgard (i33«); Alway (3^)). In 

 most soils this falls between 14 and 3. 



The upper limit for the capillary water is determined by 

 allowing a column of soil saturated with water to stand in a 

 cylinder with a perforated base till all drainage has ceased, the 

 soil being meanwhile protected from evaporation (Hilgard 

 (133a)). The percentage of water finally retained is called the 

 water-holding capacity under the conditions of the experiment. 

 It is difficult to determine exactly, but it usually varies between 

 15 and 40. 



These distinctions were drawn up in the first instance 

 when soil was supposed to have a sand grain structure and 

 when considerable importance was attached to the capillary 

 films surrounding the particles. Some modification seems 



