wherry: reactions of soils 591 



It seems evident thtit conclusions as to the requirement of 

 alkaline or acid conditions by a given species can be depended, 

 upon only when based on actual investigation of the soils in 

 which they grow. 



There are two methods in use for determining the reactions of 

 soils: titration, which shows the quantity of acid or alkali pres- 

 ent, and measurement of hydrogen-ion concentration, which 

 furnishes information as to the strength of the acid or alkali. 

 The latter feature being the more significant from the biochem- 

 ical point of view, 3 attention has been confined to it. 



The following procedure has been adopted for making the 

 observations : A sample of soil a gram or two in weight is shaken 

 up thoroughly in a thick- walled test-tube 1.5 cm. in diameter 

 with about 5 cc. of the purest water available and allowed to 

 stand at an angle of 45 degrees until the bulk of the suspended 

 matter has settled. One cc. of this liquid is pipetted off and 

 placed in a 1 cm. wide test-tube, and to it a like volume of an 

 indicator solution is added. In another 1 cm. tube 1 cc. of the 

 same indicator is diluted with 1 cc. of water. In order to over- 

 come the effect of the remaining turbidity, an additional portion 

 of the soil extract is diluted somewhat in a 1.5 cm. tube of the 

 same length as the narrower one containing the indicator, and 

 the latter tube is plunged into the cloudy liquid. The effect of 

 the turbidity on the color of the indicator is thereby rendered 

 the same in both tubes, but in the one case the indicator and 

 soil extract are mixed, in the other they are separate, and the 

 slightest change of color produced by Sbiij acid or alkali in the 

 soil extract may be readily seen on comparison of the two tubes. 



The indicators covering the extreme range likely to be met 

 with in ordinary soils, their color changes, and the intensities of 

 acidity or alkalinity corresponding to each color, are listed in 

 table 1. Instead of describing the reactions by the rather 

 difficultly comprehensible exponential method, as is ordinarily 

 done by physical chemists, direct numerical intensities are here 

 emploj^d. According to this plan the more intense the acidity, 



^ See Gillespie, Journ. Wash. Acad. Sci. 6: 7. 1916; Sharp and Hoagland, 

 Jo^lrn. Agr. Res. 7: 123. 1916; Clark and Lubs, .Town. Bact. 2: 109. 1917; etc. 



