IV. . THE REACTIONS OCCUEEING IN SOILS 57 



opinion, erroneous, for nothing analogous to tubes filled with water 

 exists in a fertile soil except, perhaps, in the interstices of compound 

 particles (clods), made up of very small particles cemented together. 

 In no case, probably, can these capillary tubes be of any great length. 

 The particles of a soil rest upon each other, leaving interspaces, which, 

 if the soil is to be fertile, must be filled with air. The water present 

 in a soil exists mainly in the form of thin films surrounding the par- 

 ticles, but not filling the interspaces. 



The rise is chiefly due to the movement of the water in these films, 

 and is greatly facilitated and influenced by changes in the forms of the 

 free surfaces of the water. 



Let two spherical particles of soil, each coated with its thin film of 

 liquid water, be brought into contact. At the point of contact the 

 water films will have a concave surface, and the surface pressure will 

 consequently be less there than on the convex surface surrounding the 

 particles ; consequently the water will move from the films around the 

 particles until the curvature of the concave surface becomes less, and 

 consequently the surface pressure there becomes greater. The water 

 will thus tend to accumulate around the points of contact of the soil 

 particles, being held there by a surface-pressure effect similar to that 

 which causes the rise of water in a narrow tube. 



If a number of particles all wetted with water are in contact, and 

 some of them lose water by evaporation or root absorption, the con- 

 cave curvature of the surfaces of water at their points of contact will 

 become greater, and thus the surface pressure will be locally decreased. 

 Consequently, water will be forced from the wetter particles where the 

 concavity of the surface is less, and where, therefore, the surface pres- 

 sure is greater (see Fig. 3). 



Thus water moves always 

 towards the portion possessing 

 the greatest concavity of sur- 

 face until equilibrium is at- 

 tained with gravitation or other 

 force acting upon it. 



The motion due to this FlG . 3 ._ Mo tion of water on soil particles, 

 cause may be upward, down- 

 ward, or lateral ; but since the greatest loss usually occurs at or near 

 the surface, and the supply of water the water table is below, the 

 upward movement is generally the most important. 



In irrigation, the lateral movement is often important, and in an 

 already moistened soil, subsequent water, e.g., rain, is carried down- 

 wards, partly by this action, though mainly, perhaps, by gravitation. 



Fig. 4 represents, in a diagrammatic manner, the constitution of a 

 soil under normal conditions. The particles of sand, etc., are represented 

 by the shaded areas, the water present as films around the particles by 

 black lines, the air spaces in the interstices of the soil, by white areas. 

 Compound particles or small clods are indicated by a difference in the 

 shading, and in these compound particles the interstices probably are 

 sometimes completely filled with water, which, in the particle, may 

 exhibit true capillary phenomena. The water table, i.e., the surface 



