WATER CONTENT AND SOIL STRUCTURE 



105 



able soil structure will result depends upon the combi- 

 nation of factors in operation. These structural modi- 

 fications have to do primarily with the finer-textured 

 soils the loams, silts and clays, rather than with 

 the sandy soils. The structure of the latter can not be 

 greatly changed. 



44. Variation in moisture content. The alternate 

 wetting and drying of a clay or a loam soil tends to 

 produce a granulated structure. It has been suggested 

 by Whitney that this is due to the contraction of the 

 moisture film around the 

 particles, as it is reduced 

 in drying. The very con- 

 siderable pressure of the 

 moisture film and the re- 

 duced friction due to the 

 presence of moisture in the 

 mass, causes the particles 

 to be drawn together in 

 small masses. This process 

 is well illustrated by Fig. 

 30, which was made from a 

 micro-slide in which was 

 mounted a suspension of 

 fine clay in water. The 

 water slowly evaporated 

 from under the cover, and 

 at last disappeared along 

 the dark lines which are 

 formed by the concen- 

 tration of the particles by 



Fig. 30. Photo-micrograph, show- 

 ing the distribution of soil particles by 

 a water film. A small quantity of clay 

 was suspended in water on a micro- 

 slide and sealed in with balsam. Evap- 

 oration was permitted to take place very 

 slowly through a small opening. The 

 retreat of the water to the dark border 

 line assembled the soil particles so that 

 they were left, to form the dark lines 

 when their mass became too great to be 

 moved by the surface tensiqn of the 

 liquid. This illustrates the granulating 

 influence of a contracting water film, 

 which is the primary force in operation 

 during the drying-out of a wet soil. 

 Note also the uniform curvature of the 

 film, as indicated by the arrangement of 

 the soil particles. 



