252 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



are valueless in connection with any study of soils in relation to 

 plant growth. 



Of the methods listed under 2 A, in Table 5, the lime-water 

 (Veitch) method gives directly the amount of lime needed to bring 

 the soil to the degree of acidity or alkalinity shown by the indicator 

 used. If phenolphthalein is selected, the result is the amount of 

 lime which will give the soil a specific alkalinity of 30. If, however, 

 litmus or brom-thymol blue were to be used, the amount of lime 

 required to render the soil neutral would be obtained. This method, 

 then, though obviously not permitting the determination of soil 

 acidity, is a real lime-require?nent method. 



The method given under 2 B, in Table 5, permits the determination 

 of the quantity of acid present in a given soil in the simplest possi- 

 ble way. An indicator, whose color changes occur at the neutral 

 point, should, however, be used. Neither methyl orange nor phenol- 

 phthalein, however useful they may be for obtaining comparative 

 results in ordinary titrations, shows when a solution has been actu- 

 ally neutralized. When an acid solution is being titrated, the former 

 changes its color long before the neutral point has been reached and 

 the latter does not begin to change until well past neutrality. 



For reasons already explained in detail, the quantity of acid pres- 

 ent in a soil is far less significant than the hydrogen-ion concentra- 

 tion or specific acidity, when effect upon the growth of plants is 

 under consideration. The methods under 2 C, Table 5, are, therefore, 

 the only ones which should be used in studying this subject. Method 

 a is difficult to apply and to interpret. Method o is justly resorted to 

 for testing soils where the necessary apparatus is available. 



The indicator method c, 4 while less accurate than the potentiom- 

 eter method, is, however, so simple to apply that it can be recom- 

 mended for use in all ordinary studies of soil acidity in relation to 

 plant growth. 



A FIELD METHOD FOR MEASURING SOIL ACIDITY." 



Under the most favorable conditions it is possible by the indicator 

 method to measure acidity and alkalinity with much greater pre- 

 cision than is necessary in the field. By treating the indicators with 

 buffer solutions of known ionic concentration, many hues intermediate 

 between those in the accompanying color chart 6 can be distinguished. 



* Clark and Lubs, Jour. Bacteriology, 2, 1, 1917. Gillespie, Jour. Wash. Acad. Sci., 6, 7, 

 1916. 



5 First published in Jour. Wash. Acad. Sci., 10, 217-223, 1920; reprinted with minor 

 modifications in Ecology, 1, 170-173, 1920 ; here copied from the latter, with footnotes 

 added on pp. 252-254. 



a The color chart previously published (Ecology, vol. 1, facing p. 172, 1920) was copied 

 from layers of indicator solutions, brought to the different degrees of acidity and alka- 

 linity, 1 cm. thick. The present one, which is intended primarily for use with a porcelain 

 plate containing depressions 2 or 3 mm. deep, in which the colors are viewed, shows the 

 colors decidedly paler. The colors of the solutions as shown in such a plate have been 

 kindly copied, in oil paints, by Mr. J. Marion Shull, of the Bureau of Plant Industry, and 

 are here reproduced from his chart by lithography. 



