Studies in Soil Physics. 33 



air surface. On this curved surface there will exist (according 

 to well-known laws) a surface tension tending to pull the sides 

 of the wedge together, or in the case of the soil grains, to hold 

 them together at the points of contact. It is these water-film 

 forces which control the formation and persistence of soil ag- 

 gregates and enable the preservation of more open structure than 

 could endure in the dry soil. They are the determiners of the 

 structure. * Without them soils would tend to assume, ard in 

 most cases would assume, the closest structure (and therefore 

 texture) which their mechanical composition would allow. 



Furthermore, the nature and strength of these water-film 

 forces will depend upon the water content of the soil. The 

 absolute value of the surface tensions in the wedge-shaped inter- 

 spaces will depend upon the curvature of the water-air surface, 

 and it is obvious that this curvature will alter as the surface re- 

 treats toward or advances from the point of solid contact — that 

 is, as the water films generally decrease or increase in thickness. 

 Now, for a given mechanical composition, the thickness of the 

 water films is directly dependent upon the water content — 

 hence the intimate relation of water content and structure. If 

 a soil be so dry that water films do not exist or are not generally 

 in mutual contact, the surface tension forces will be absent and 

 the structure will depend upon gravity alone. If, on the other 

 hand, the soil be saturated or nearly so, the interspaces will be 

 full of water, the water-air surface and its tensions will disappear 

 and structure will again be controlled in the main by gravity. 

 Over water contents intermediate between dryness and satura- 

 tion water-film forces will be present and more or less in control, 

 and it is apparent that at some one of these intermediate water 

 contents the water-film forces will have their maximum strength 

 and efficiency and the soil structure (and texture) will be the 

 most open possible. At this point — called the "critical moisture 

 content" — each of the physical properties of the soil (when 

 plotted against water content) shows either a maximum or a 

 minimum. % The openess of texture is, as already stated, at a 

 maximum, and since, other things equal, the maximum openness 



*ChemicaIly cemented and other relatively permanent aggregates of soil particles are of 

 course excluded from consideration. In their relations to physical condition these 



aggregates behave like solid particles. 



JThis fact was experimentally discovered before the development of the water-film theory 

 of soil structure. See Cameron and Gallagher, Bull. 50, Bureau of Soils, U. S. Dept. 



of Agriculture (1907). 



