60 The Plant World. 



gravitational, accordingly as it is (1) held in films about the 

 individual soil grains, (2) held by surface tension in the capillary 

 interspaces of the soil, and (3) moving freely under the action of 

 gravity. The statement of this classification in most textbooks 

 is unfortunate in that it suggests, if it does not afiirm, the exist- 

 ence of the soil water in three diff"erent states or conditions, dif- 

 fering in properties and in usefulness to the plant. This is quite 

 erroneous. The classification is one of forces, not of ma- 

 terials or states, and all the acting forces afi"ect directly or indi- 

 rectly all of the soil water. It will be better to drop entirely 

 the old terminology, consider the soil water system as a unit, 

 and speak not of hygroscopic, capillary and gravitational 

 water, but of hygroscopic, capillary and gravitational 

 forces. The condition and behavior of the soil water under 

 any given circumstances will depend upon forces belonging to 

 all three of these classes and will be determined by their re- 

 sultant, not any one of them alone. 



Hygroscopic forces. The hygroscopic forces are 

 referable directly to the adhesion (whatever that may be) be- 

 tween liquid water and the soil grains. This adhesion is very 

 strong. Thin films of water are (by some mechanism unknown) 

 held very tenaciously by the solid particles of the soil. The 

 measure of this effect, or the hygroscopicity, is of well recognized 

 importance as a soil constant, but as such it will be discussed in 

 a later paper. The importance of the hygroscopic forces for 

 the present discussion lies in that they furnish the attachment 

 or fulcrum for the action of other forces. By them the soil 

 water system is bound to the solid framework of the soil. Ob- 

 viously water stays in the soil at all only because of the adhesion 

 between it and the soil grains, and it is equally obvious that this 

 adhesion is the strongest of the mechanical forces which affect 

 the system. Capillary or gravitational activities are never able 

 to pull loose the water films from the solid grains. 



The capillaiy movement of soil water. * 

 In the previous paper of this series it was pointed out that in a 

 moist but unsaturated soil the water exists in the form of films 

 about the grains, which films are in union where the solid soil 

 grains touch each other. It was explained that strong surface 

 tension forces exist on the curved water-air surfaces at these 



*The discussion of this section is drawn largely from the papers of Briggs (Bull. 10, Bureau 

 of Soils, V. S. Dept. of Agriculture, 1897) and Buckingham (Bull. 38, ibid, 1907). 



