122 BULLETIN 1059, U. 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 coUoids for calcium ion 
under 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. 
(o) 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. 
&. 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 
