404 Journal of Agricultural Research vol. xx. No. 5 



soil is considered to be the result of the formation of certain new silicates. 

 Possibly in soils leached with water and not containing an excess of salts 

 the dispersed condition may be the result of the almost complete removal 

 of electrolytes from the films of solution surrounding the soil particles. 

 To a lesser extent the s?,me thing occurs when the soil solution is depleted 

 through absorption of solutes by the plant. None of the data presented 

 in this paper, however, are of such a nature as to permit of any conclu- 

 sions with regard to these very difficult questions concerning the colloid 

 chemistry of the soil. 



Neither is it possible to state definitely the effects of the fluctuating 

 soil solution on the physical state of the soil under field conditions. 

 A sample of soil may be maintained in a relatively pervious state even 

 after long washing, provided the compound particles of soil are not dis- 

 turbed by stirring or mixing while the soil is saturated with moisture. 

 Nevertheless, it is probable that the soil in the field is subject to certain 

 modifications in its physical state which are merely accentuated when the 

 laboratory tests are carried out. 



It is interesting to speculate on the indirect effects of the changes in the 

 physical condition of the soil noted in these experiments. It is entirely 

 possible that such changes may be of considerable importance. The 

 aeration, resistance to root penetration, ease of cultivation, percentage 

 of unfree water, etc., are very probably affected to a greater or less degree, 

 and these alterations in the soil conceivably may have an important 

 influence on the growth of microorganisms or plants. In any case, it is 

 highly desirable to make observations on all the effects, direct and indi- 

 rect, which may be correlated with the changing concentration or com- 

 position of the soil solution. It should be strongly emphasized that in 

 studies of soil fertility the whole system of soil, soil solution, and plant is 

 so constituted that all the components must be considered as interrelated. 

 Thus, the plant may exhaust the soil solution with a resultant change in 

 physical condition of the soil which may be unfavorable to the growth of 

 microorganisms, and this inhibition in time may influence the concentra- 

 tion of certain solutes in the soil solution. It is believed that the greatest 

 advances in theories of soil fertility will come with an extension of our 

 knowledge of the soil solution in its dynamic aspects. 



CONCLUSIONS 



The physical state of certain soil constituents is influenced to a marked 

 degree by the concentration of the soil solution. The colloidal state of the 

 soil suspension undergoes significant alterations during the season. A 

 large increase in colloidal matter is noted when the soil solution is de- 

 pleted as a result of absorption of solutes by the plant. 



