122 BULLETIN 1059, IT. S. DEPARTMENT OF AGRICULTURE. 



the soil by potassium nitrate solution, a method which has been 

 much used and debated, but which should probably from now on 

 be considered only for its historical interest. It has many times 

 been found that this method produces appreciable acidity in soils 

 which at the same time evidence alkalinity. 



2. No salt solution, but some pure water, is added to the soil. 



(a) The mixture is titrated with lime water, using either an indicator or 

 observation of the freezing point to determine the end point. This gives the 

 amount of lime needed to neutralize the acid originally present in the soil plus 

 that produced indirectly by the action of lime (which is likely to differ from 

 that produced by a neutral salt solution) as well as the amount of lime 

 required to satisfy the absorptive power of the soil colloids for calcium ion 

 iinder the given conditions. 



The determination of the end point in such a water mixture by 

 the freezing-point method is the method described by Bouyoucos 

 (108), and is based on the fact that as long as the CaOH added is 

 combining with a free acid or an acid salt (which is up to the point 

 of neutrality), the solution will contain fewer and fewer ions, and 

 consequently will have a higher and higher freezing point. When 

 the CaOH molecules begin to remain in solution, however, there, is 

 an immediate change in the opposite direction. This method appears 

 to have considerable value, though not wholly a measure of the free 

 acids. Likewise, when the reaction of the solution has been shown 

 not to be acid, through an immediate lowering of the freezing point 

 on adding CaOH, it would seem that the normal freezing-point de- 

 pression was a measure of the alkalinity. 



(6) The mixture is filtered and the filtrate titrated with standard alkali. 

 This gives the quantity of acid present in the soil. 



By titration with KHS0 4 solution, the nitrate may likewise be 

 tested for alkalinity, the method being described in the appendix. 

 It is perhaps desirable to bring out here, however, since both of these, 

 methods may be used, that the commonly used indicator, phenol- 

 phthalein, does not indicate neutrality, but a specific alkalinity of 

 30. Wherry suggests the use of litmus of brom-thymol to detect 

 complete neutrality. 



(c) The hydrogen-ion concentration or specific acidity (or alkalinity) is de- 

 termined — 



a. By catalysis of an ester. 



b. By measurement of the potential due to hydrogen-ion with the 



potentiometer. 



c. By observation of color changes of indicators whose relations to 



hydrogen-ion concentration are known. 



This last-named method is that which Wherry then describes in 

 detail. It consists primarily of the use of six indicators in various 



