SOIL ACIDITY WHERRY. 255 



tion; and this is continued until either an intermediate color of one 

 indicator, or opposing extremes of two overlapping ones, are ob- 

 tained, whereupon the specific acidity or alkalinity can be read off 

 from the chart. 



The more turbid the liquid the more indicator must be added, and 

 the less certain are the results obtained. The turbidity can, of course, 

 be removed by the addition of coagulating agents or by filtration 

 through paper; but it is essential to make certain that these do not in 

 themselves show an acid or an alkaline reaction. The most satis- 

 factory results of all can be obtained by running a quantity of the 

 extract through a paper filter until two successive portions yield the 

 same value when tested with indicators. But such procedures are 

 more suited to laboratory than to field studies, and after a little 

 experience one can tell the indicator color change with certainty, 

 even in the presence of considerable brown mud. 



To illustrate the procedure followed in actual practice, two typi- 

 cal cases encountered by the writer may be cited here. 



1. A black soil in pockets in limestone rock, supporting spleen- 

 wort ferns, was treated as above, and on testing the soil extract with 

 bromthymol blue indicator, a strong blue color was obtained ; refer- 

 ence to the chart showed that the reaction must be alkaline, and the 

 value of specific alkalinity 3 or more (Ph=7.5). The process was 

 repeated with the indicator, the color changes of which lay next to- 

 ward the alkaline side of the table, namely, phenol red. With this 

 indicator a clear red color was obtained, showing the reaction to be 

 actually specific alkalinity 10 (Ph=8.0). 



2. Soil from a dry blueberry thicket was tested, and, since upland 

 peat is usually distinctly acid, the first indicator tried was brom- 

 cresol purple, the color changes of which occur just to the acid side 

 of the neutral point ; with this indicator a yellow color was obtained, 

 indicating a specific acidity of at least 30. The soil was accordingly 

 tried again with methyl red, which lies next toward the acid side, 

 and this gave a violet-red color, corresponding to a specific acidity 

 of 300 or more. It was accordingly necessary to try an indicator 

 working at still higher acidities, namely, bromphenol blue ; and this 

 yielded a violet color, indicating 300 or less. The last two indicators 

 agreed, then, in fixing the reaction of this soil as: specific acidity 

 300 (Ph=4.5). 



In spite of certain limitations, this method is capable of giving 

 definite information as to soil reaction in many cases. And the re- 

 sults obtained by the writer on a number of species of native plants 

 have been of such significance that the method is published for the 

 benefit of students of plant distribution and others interested in soil 

 acidity and alkalinity. 



