SOIL ACIDITY WHEERY. 253 



On comparing the colors thus produced with those developed by 

 mixing clarified soil extracts with the same indicators, specific 

 acidities differing by a factor of 5 yi0 or 1.59 (P H = 0.2) can be 

 recognized. In the field, where it is inconvenient to carry buffer 

 solutions to prepare standards for comparison, and where the 

 turbidity of soil extracts is difficult to remove, it is impracticable to 

 work closer than values differing by a factor of yiO or 3.16 

 (P H =0.5) which is rounded off for simplicity to 3-f-. This degree 

 of precision is, however, entirely adequate for the purpose in view, 

 for it has been repeatedly found that from one to another plant of 

 the same species, or indeed, from one to another root on the same 

 individual, separate observations of reaction may differ by a factor 

 of 10 or more. 



The following outfit is used: 7 First, a rectangular box about 3.5 

 by 5 by 9 centimeters in dimensions. In the box, six vials for the in- 

 dicators, 1.5 bj 7 5.5 centimeters, capacity 8 cubic centimeters, each 

 provided with a cork or rubber stopper, into which is inserted a glass 

 rod flush with the top of the stopper, and extending nearly to the 

 bottom of the vial ; to prevent undue compression upon inserting the 

 stoppers, a groove may be cut in the side of each, so as to reach nearly 

 to the lip of the vial. Then, three or four vials, in which to extract 

 the soils, about 2 by 5 centimeters, made of heavy glass, to prevent 

 undue breakage ; a container for water, which may conveniently be a 

 screw-capped jar holding 200 cubic centimeters or more, or an 

 aluminum canteen; and a pipette, most simply constructed of two 

 pieces of glass tubing a few centimeters in length, connected by a 

 rubber tube. 



The six indicators which have proved most satisfactory in work 

 with soils are: Bromphenol blue, bromcresol purple, bromthymol 

 blue, phenol red, methyl red, and 0-cresolphthalein or phenolphthai- 

 ein. The first three are used, as recommended by Clark and Lubs, 

 in about 1 per cent solution in water, titrated with dilute sodium 

 hydroxide to their intermediate colors; and the phenol red in a 0.5 

 per cent solution similarly titrated. The methyl red and phenol- 

 phthalein are used as 0.25 per cent solutions in 50 per cent alcohol. It 

 should be noted here that litmus paper, which is often recommended 

 for testing soil reaction, is much less sensitive than the above indi- 

 cators, and may give misleading results. 8 



A simplification of the procedure previously recommended has 

 been adopted; modifications may still be desirable in special cases. 

 But before giving the directions, a word should be added concern- 



7 Sets of indicators similar to that here described are for sale by the La Motte Chemical 

 Products Co., 13 West Saratoga Street, Baltimore, Md. 



8 Gillespie and Wise, Jour. Amer. Chem. Soc, 40, 796, 1918. 



