12 gillespie: hydrogen-ion concentration in soils 



against an accurate potentiometer with the kind assistance of Dr. 

 William Mansfield Clark, of the Bureau of Animal Industry. 



In order to see whether a substantially correct potential had been 

 obtained, the shaking was continued after the first measurement for 

 5 to 9 minutes longer and the potential was determined again in the 

 same way. This was done in all cases but one. In 7 cases the poten- 

 tial observed was the same, in 10 cases it had fallen (1 fall of 3, 1 of 2, 

 and 8 of 1 millivolt), and in 4 cases it had risen (1 rise of 3, and 3 of 1 

 millivolt). No relation could be seen between the changes and the 

 values of the potential. A possible interpretation of the falls of po- 

 tential is that a more complete saturation of the water with acid sub- 

 stance was attained during the second shaking. The rises may have 

 been due to hydrolyzation of soil minerals. No difficulty was encoun- 

 tered even with the measurement of soils which gave neutral or alkaline 

 results. 12 



COLORIMETRIC DETERMINATIONS 



Fifteen grams of dry soil were treated in a centrifuge tube of 60 cc. 

 capacity with 30 cc. of distilled water. After thorough wetting of the 

 soil was accomplished by means of shaking or of slight stirring the tube 

 was closed by the hand and violently shaken fifty times. Not more than 

 8 soils were thus treated at a time. The entire determination was now 

 carried out without delay. The tubes were centrifuged for 10 to 20 

 minutes. The supernatant fluid in some cases was almost clear, in 

 many cases there was considerable turbidity, and in a few cases there 

 was a heavy turbidity together with a yellowish color. With a pipette 

 provided with a rubber tube and mouthpiece 15 to 20 cc. were with- 

 drawn, and 5 cc. were put into each of 3 test-tubes. Indicator solu- 

 tion was added and admixed and the color so developed was compared 

 with the colors obtained on adding the same quantity of the same in- 

 dicator solution to tubes containing 5 cc. of various "regulators" of 

 known hydrogen-ion concentrations. With all fluids it was possible to 

 make a satisfactory comparison by means of at least one indicator, and 

 with most fluids it was possible to make an independent comparison 

 with a second indicator. This was done whenever possible. When 

 the color which developed in the soil extract was the same as that in a 

 particular regulator, the hydrogen-ion exponent of that regulator was 



12 In preliminary experiments the use of soil and water in such quantities that 

 agitation of the mass during shaking was inefficient was found to prevent the 

 attainment of approximately equilibrium conditions within five minutes. 



