wherry: the habitat of the walking fern 675 



an excess of water to a soil will bear any simple relation to that 

 of the original soil liquid, nor that it will contain any definite 

 fraction of the total soluble lime. Although the analytical 

 results will, therefore, have no absolute numerical significance, 

 their general order of magnitude should show in a rough way the 

 extent to which lime is available to the plant. In order that the 

 results should represent as nearly as possible the natural con- 

 dition of the soil, the samples to be used for determination of 

 soluble lime were not dried, finely sifted, or pulverized. For 

 the same reason water saturated with carbon dioxide was used 

 for extracting, and the amount of this water was reduced to the 

 smallest possible quantity consistent with convenient analytical 

 operation. 



One-gram samples of soil were carefully separated from the 

 roots and placed in centrifuge tubes, 3 cc. of distilled water 

 saturated with carbon dioxide being added; the tubes were 

 shaken gently to insure uniform moistening, stoppered, allowed 

 to stand one hour, and centrifuged for a few minutes, and the 

 lime in the liquid was determined in the usual way. The results 

 obtained are given, along with the others, in Table 1. 



Still another feature of the soils, their acidity or alkalinity, 

 seemed worth determining, since there is evidence that some 

 plants are sensitive to relatively slight changes in these factors. 

 The acidity or alkalinity of soils is often expressed in terms of 

 "normality," the equivalent weight of soil being taken as 1000 

 and water extracts being titrated with standard alkali or acid, 

 using phenolphthalein as indicator; and such normality determi- 

 nations were made as described below. In addition, to determine 

 the true acidity (or alkalinity), the soils were examined by 

 the colorimetric method developed by Dr. L. J. Gillespie of 

 the Bureau of Soils, U. S. Department of Agriculture. 3 The 

 results obtained by this method are presented in two forms, in 

 the last three columns of the table. First the P H value is given, 

 this being the negative exponent of 10 corresponding to the 

 concentration of hydrogen ions present in the solution. Then, 

 as the P H figures do not express clearly the relative strengths 



3 Journ. Wash. Acad. Sci., 8: 7. 1916. 



