56 THE SOIL SOLUTION 



wards of 1,000,000 ohms. The soil or rock powder under ex- 

 amination is then added to the cell, being rapidly stirred into 

 the water contained therein. The resistance drops to about 

 5,000 ohms with a short space of time, usually three or four 

 minutes. A further slight drop in the resistance generally takes 

 place, but it requires days, and sometimes even months to be- 

 come more than barely appreciable. In this manner it has been 



Time 



Fig- 3- 



shown that the soil and many of the common soil minerals 

 dissolve quite rapidly if they are sufficiently fine to offer a large 

 surface to the action of the water. It would seem to follow, 

 therefore, that in the case of the soil solution the concentra- 

 tion with respect to these constituents derived from the soil 

 minerals, will be rapidly restored whenever disturbed through 

 absorption by plants, leaching, or otherwise. 



That the minerals of the soil, or a powdered mineral or rock- 

 powder, will dissolve continually as the concentration of the so- 

 lution in contact with it is disturbed by abstraction of a dis- 

 solved mineral substance, has been shown by numerous experi- 

 menters. An apparently obvious way to test this point would be 

 to treat che soil sample with successive portions of water, and 

 to analyze the successive portions for the dissolved mineral sub- 

 stances. This method, however, involves serious experimental 

 difficulties, owing to the smaller sized mineral particles being 

 suspended in the mother liquor, thus precluding satisfactory de- 

 cantation and clogging filters. Moreover, such a process in no 

 case simulates field conditions. To meet these difficulties, the 

 soil or mineral powder has been placed between two porous 



