68 



IRRIGATION PRACTICE 



are often as high as 50 to 75 per cent of the original 

 volume; with average soils, receiving moderate quanti- 

 ties of water within the limits of practical agriculture, 

 the volume changes are from 7 to 12 per cent of the 

 original volume. Such fairly large variations, occurring 

 over acres of land, represent great total changes, capable 

 of modifying deeply the character of the soil. 



The reason for such volume changes is simple. In a 

 dry soil the particles, lying rather closely side by side, 

 occupy a relatively small space. When water is added, 

 the soil particles group themselves into larger loose aggre- 

 gates or crumbs, which occupy more space. There is a 





FIG. 16. Cracked river sediments showing volume changes due to water. 



continuous arrangement and rearrangement of soil parti- 

 cles, and a corresponding variation in the soil volume as 

 the percentage of water in the soil changes. 



Cameron and Gallagher found that, as water is added 

 to the soil, the volume becomes larger and larger, until a 

 certain definite point is reached, after which the volume 



