256 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1920. 



THE RELATION OF SOIL ACIDITY TO PLANT DISTRIBUTION. 

 INTRODUCTORY NOTE. 11 



As a result of many hundreds of determinations of soil acidity 

 and alkalinity made by the above-described method upon plants 

 growing under the widest range of physical and climatic conditions, 

 the writer has found abundant evidence that the acidity of the soil 

 is closely connected with the distribution of native plants. It is 

 not intended to imply that the reaction is the only factor of impor- 

 tance in determining the place of growth, nor that the acid or alkali 

 necessarily acts directly on the plant. Some plants may require for 

 themselves or for symbiotic organisms a soil of a definite acidity 

 (or alkalinity) ; but others may be favorably affected by some physi- 

 cal or chemical property of the soil which accompanies the develop- 

 ment of that acidity ; and still others may be driven into soils of a 

 certain degree of acidity by more vigorous species which monopolize 

 neighboring soils of greater or less acidities. The measurement of 

 the actual soil acidities and alkalinities connected with certain 

 species of plants, which is all that is attempted in the present series 

 of studies, is but one step in the working out of the problem of why 

 a given plant grows in a certain place. It is hoped that the results 

 presented will indicate, however, the considerable, if not funda- 

 mental, importance of this heretofore often neglected step. 



STUDIES ON FERNS. 



Rock ferns. 12 — Judging from the literature, the ferns which grow 

 on rocks would appear to be, on the whole, markedly sensitive to the 

 chemical features of their soils. Their distribution is, of course, 

 controlled to some extent by physical factors, such as climate, 

 porosity of soil, availability of moisture, etc. ; yet in many instances 

 a given species has been observed to grow in soils of widely varying 

 physical character, but consistently associated with a particular type 

 of rock, and accordingly more or less uniform in chemical composi- 

 tion. Again, soils of like physical properties but dissimilar chem- 

 ical nature often occur in such proximity that spores of the various 

 ferns can not fail to have fallen into both kinds, yet flourishing plants 

 have developed in but one of them. 



It is commonly recognized that certain species of rock ferns grow 

 by preference upon limestone and similar rocks, and are accordingly 

 to be classed as calcareous soil plants. Other species, however, ap- 

 pear to avoid calcareous rocks quite definitely, and are presumably 



11 Reprinted with minor changes from introduction to an article in Ecology, 1, 42, 1920. 



12 Abstracted from " The soil reactions of certain rock ferns," Amer. Fern Journ., 10, 

 15-22 and 46-52, 1920. Slight changes have been made in the table as the result of fur- 

 ther work since the original paper was prepared. 



