Chapter V. 



THE DYNAMIC NATUKE OF SOIL PHENOMENA. 

 In soil investigations, until recently, the assumption has been 

 made, more or less explicitly, that any given soil mass, as for 

 instance a field, remains fixed or in place indefinitely. It has 

 been admitted, of course, that some physical, chemical and 

 biological processes might be taking place in the soil, but these 

 have been regarded as relatively unimportant in their effects 

 upon the soil mass in toto. It has been assumed that the only 

 important change taking place in the soil is a loss of mineral 

 plant nutrients, partly by leaching, partly by removal in the 

 garnered crops. In other words, the soil has been regarded as 

 a static system. This is a fundamental error. In studying the 

 soil as a medium for crop production, we must consider the 

 plant itself, or at least that part of the plant which enters the 

 soil, namely, the root; the solid particles of the soil; the soil 

 water, or the aqueous solution from which the plant draws all 

 the materials for its sustenance, excepting the carbon dioxide 

 absorbed by its aerial portions; the soil atmosphere; the biologi- 

 cal processes taking place. The one common characteristic of 

 all these things is that they are continually in a state of change; 

 therefore the soil problem is essentially dynamic. 



The root of a growing plant is always moving. 1 The amount 

 of motion may be small or large, depending upon the surround- 



1 In order to penetrate the soil, a living root must be capable of exert- 

 ing large pressures, and indeed, the magnitude of these pressures has been 

 determined for some cases. See, for citations of the literature, Pfeffer, 

 Plant Physiology, translated by Ewart, 1903, Vol. 2, p. 124 et seq. But 

 it cannot be doubted that, in general, root movement is much facilitated 

 and perhaps directed by movements among the soil particles. As the 

 absorbing tip of the root removes film water from the adjacent soil grains, 

 there is a necessary rearrangement of these grains with a shrinking away 

 from the tip, which then moves forward by taking advantage of the move- 

 ments among the soil grains. 



