GILLESPIE: HYDROGEN-ION CONCENTRATION IN SOILS 15 



two methods. The soils are arranged in the order of decreasing acidity. 

 An exponent of 7 means neutrality; a smaller one, acidity; and a larger 

 one, alkalinity. It will be seen that values were obtained between 4.4 

 and 8.6. Some of the acidities found are rather intense, from a bio- 

 logical point of view. Thus Briinn 15 has found that typhoid bacilli are 

 killed with certainty by exposure for 24 hours to an acidity correspond- 

 ing to an exponent of 5, and four of the soils show as high, or a higher 

 acidity. One soil gave a neutral result and the four western alkali 

 soils gave distinctly alkaline results. 



A study of the results for each soil obtained by the use of the various 

 indicators and by the electrometric method shows that the agreement 

 between the two indicators is very good except in one instance (No. 

 17), and that the agreement between the colorimetric and the electro- 

 metric methods is good in every case. Such agreements show that the 

 two methods give comparable results and give ground for inferring that 

 such results are approximately correct. 



It would seem premature at this time to apply the results obtained in 

 this way to the soil as it exists in the field, since carbon dioxide is lost 

 during the drying and the natural "soil solution" is diluted for the 

 determinations. Nevertheless it may well happen that the errors so 

 introduced (from a field standpoint) are not great enough to obscure 

 the differences observable between different soils. Further work will 

 be required to ascertain whether this is the case. 



SUMMARY 



Procedures have been devised for the electrometric and col- 

 orimetric determination of hydrogen-ion concentration in soil 

 admixed with two parts of water. Twenty-two soils of various 

 types and reactions were examined by means of these procedures. 

 The hydrogen-ion exponents so determined were from 4.4 to 8.6. 

 Some of the values are acid, some nearly neutral, and some dis- 

 tinctly alkaline. Of the acid values some represented rather 

 intense acidity. 



In 19 cases two different indicators could be used for the 

 colorimetric test, and in these cases there was a good agreement 

 between the two results so obtained. In all cases there was a 

 good agreement between the electrometric and the colorimetric 



15 Dissertation, Berlin, 1913; quoted by Michaelis in the book cited above. 



