222 wherry: determining soil acidity 



45 ° and allowed to stand until the bulk of the suspended matter 

 has settled. The more or less clear liquid is then decanted or 

 pipetted off into another vial, a drop or two of bromthymol 

 blue or one of the other indicators, the color changes of which 

 occur near the neutral portion of the table, are added, and the 

 color assumed is noted. If either of the extreme colors is shown, 

 the process is repeated with the indicator whose color changes 

 come next in the corresponding direction; and this is continued 

 until either an intermediate color of one indicator, or opposing 

 extremes of two overlapping ones, are obtained, whereupon the 

 specific acidity or alkalinity can be read off from the table. 



The more turbid the hquid, 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 satisfactory results of all can be obtained 

 by running a quantity of the soil 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 

 typical cases encountered by the writer may be cited here. 



(i) A black soil in pockets in Hmestone rock, supporting 

 spleenwort ferns, was treated as above, and on testing the soil 

 extract with bromthymol blue indicator, a strong blue color was 

 obtained; reference to the table showed that the reaction must 

 be alkahne, 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 toward 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 



