592 



wherry: reactions of soils 



the larger the number used to describe it; thus, a hquid with an 

 intensity of 1000 is 10 times as acid as one with an intensity 

 of 100, and so on.'* 



TABLE 1 

 Indicators* for Soil Acidity and Alkalinity Determination 



* Samples of several of these were kindly supplied to the writer by Dr. H. A. 

 Lubs. 



The indicators are used in extremely dilute solutions, mostly 

 about 0.0001 per cent, although it is most convenient to trans- 

 port them into the field in more concentrated form, and to dilute 

 them as needed. For the first examination of each soil phenol 

 red is employed, as its color changes occur around the true 

 neutral point, at which, according to the plan here adopted, 

 acidity = alkalinity = 1. When the effect produced is that of 

 either its most acid or most alkaline color, the procedure is re- 

 peated with indicators showing changes over adjoining ranges 



* This plan was used by the writer in a previous paper on a similar subject, 

 Journ. Wash. Acad. Sci. 6: 675-676. 1916. Its simplicity has led to its employ- 

 ment throughout the present one, but for the benefit of those accumstomed to 

 the Ph values, they are added to the table containing the summary of results. 

 To transform numerical intensities into Ph exponents : if acid, subtract the power 

 of 10 represented by the number from 7; if alkaline, add 7 to it. The ''two 

 points of interest on the scale of hydrogen-ion exponents for soils" recently 

 discussed by Gillespie (Science 48: 393-394. 1918), namely 6.0, the acid limit 

 for Azotobacter, and 5.2, that for the potato-scab organism, would be repre- 

 sented in the scale here used by acid 10 and acid 63 respective!}'. 



