THE DYNAMIC NATURE OF SOIL PHENOMENA 19 



ing conditions or attendant circumstances, but cessation of mo- 

 tion means the death of the root. This becomes evident from a 

 consideration of the mechanism of root growth. The living 

 root absorbs and excretes water and dissolved substances through 

 a restricted area just back of the root tip or the tips of the root 

 hairs. While absorption is taking place, however, there is a 

 deposition of denser material over the absorbing area, or "root 

 corking." But coincident with the corking process, the tip is 

 pushed forward between the soil grains into the nutrient medium, 

 new cells are formed and a new absorbing surface continually 

 brought into functional activity. A failure of the plant root 

 to move forward in this way would mean a reabsorption of root 

 effluvia with harmful consequences to the plant, or a corking over 

 of the root without further formation of absorbing surface and 

 with consequent cessation of its functioning. This would mean 

 the inevitable death of the root, and, if general, of the whole 

 plant. It is clear, therefore, that root penetration and absorption 

 of plant nutrients are essentially dynamic. 



The solid components of the soil are always in motion. 

 Every soil, no matter how flat the area or how well protected 

 by vegetal covering, suffers some translocation of soil material 

 through rains, as is evidenced by suspended material in the run- 

 off waters. On hillsides this is shown by the soil accumulating 

 on the "up" sides of fences, especially stone fences. In the 

 aggregate this movement is probably quite large everywhere. 

 It is manifestly so in the watersheds of many of the world's im- 

 portant rivers as shown by their muddy waters and the forma- 

 tion of deltas, sometimes of great area and agricultural im- 

 portance. 



With the saturation or approach to saturation of the sur- 

 face soil the particles are more easily moved among themselves 

 by an extraneous force. It is very rarely that the surface of a 

 field is a dead level. Consequently when the soil is wetted, the 

 gravitational force on the individual soil grains produces a more 

 or less pronounced "creeping" effect down hill. On decided 



