THE DYNAMIC NATURE OF SOIL PHENOMENA 23 



and mainly under the influence of gravity. For convenience this 

 downward-moving water is designated as "gravitational" water. 

 It moves through the soil with comparative rapidity and a por- 

 tion reappears elsewhere as seepage water, springs, etc. But 

 with the return of fair-weather conditions at the surface, there 

 is increased evaporation and augmentation of the fly-off, and 

 there is developed a drag or "capillary pull" on the water below. 

 A large portion of the cut-off thus returns to the surface, main- 

 ly through films over the surface of the soil grains and in the 

 finest interstices. 1 



The soil atmosphere is continually in motion, following with 

 more or less decided lag the barometric changes in the atmos- 

 phere above the soil. Moreover, the chemical and physical pro- 

 cesses continually taking place in the soil involve the absorption 

 or the formation of free carbonic acid, and it seems probable that 

 all rain water penetrating the soil gives up some oxygen to the 

 soil atmosphere. The bacteria and lower life forms are nec- 

 essarily undergoing changes continually. In fact all compo- 

 nents of the soil are continually undergoing, or are involved 

 in, changes of one kind or another. 



It is certain that investigation of the various motions and 

 changes taking place in the soil is quite as important as investi- 

 gation of the soil components, and that no clear idea of the 

 chemistry of the soil can be obtained without it. The develop- 

 ment of a rational practice of soil control is possible only when 

 the soil is regarded from a dynamic viewpoint. 



1 Leather, however, thinks the water returns from only a limited depth, 

 some 5-7 feet; see, The loss of water from soil during dry weather, by 

 J. Walter Leather, Memoirs of the Department of Agriculture, Agricul- 

 tural Research Institute, Pusa, India, Chemical series, 1, 79-116 (1908). 

 Dr. George N. Coffey has called the author's attention to some observa- 

 tions in Western Kansas, where a prolonged drought had dried the soil 

 to a considerable depth. A fairly heavy rain wetted the soil to less than 

 two feet from the surface, and practically all of this moisture had returned 

 to the surface and evaporated within a few days. Such special cases as 

 these, however, interesting in themselves, are even less so than the normal 

 cases in humid areas, where a part of the water passes through the soil 

 as seepage, the larger portion returning to the surface, sometimes through 

 distances of many feet. 



