PAPERS OX CHEMISTRY AND PHYSICS 2S1 



these and other factors contribute in varying degrees to 

 the results obtained by the application of various acidity 

 methods to soils, these various methods can not be ex- 

 pected to give concordant results. 



It is the purpose of this paper to record the results 

 obtained in a comparative study of five different soil 

 acidity methods, as applied to a number of Illinois soils 

 which vary rather widely in type and in lime-require- 

 ment. 



THE METHODS 



Three of the methods used, those of Hopkins. Veitch 

 and Jones, are recognized as quantitative. The Comber 

 and Truog methods, which were designed primarily as 

 qualitative tests for field use, furnish also a rough index 

 of the degree of acidity. 



The Hopkins method consi-ts essentially in shaking 

 100 grams of soil continuously for three hours with 250 

 cc. of normal potassium nitrate solution. After settling 

 has taken place, 125 cc. of the solution are decanted and 

 titrated with standard alkali, after boiling to remove 

 carbon dioxide. From the titration value and a factor 

 proposed by Hopkins, the linie-requirement is com- 

 puted in terms of calcium carbonate equivalent. 



In the Veitch method, separate 10 gram samples of soil 

 are treated with increasing amounts of standard calcium 

 hydroxide solution and evaporated slowly to dry: a - 

 Then, after a short digestion on the steam bath with dis- 

 tilled water, a portion is filtered off and boiled down 

 nearly to dryness and tested with phenolphthalein. From 

 the amount of Ca(OH), used in the sample which is neu- 

 tralized, is computed the lime requirement. 



The Jones method consists in grinding a small sample 

 of soil, dry, in a mortar with a weighed quantity of cal- 

 cium acetate. When thorough mixture is effected, dis- 

 tilled water is added, the whole stirred for one minute, 

 filtered and titrated with standard alkali solution. 



It will be noted that the Hopkins and Jones methods 

 depend upon reaction with a neutral salt for liberation 

 of acid-reacting compounds from the soil, while the 

 Veitch method measures the absorption capacity of the 

 soil for a free base. 



