May 26, 1922] 



SCIENCE 



569 



plants. The Danish eeologist Warming, it is 

 true, distinguished a group of "oxylophytes" — 

 acid place plants — but he had few followers. 

 Coville'^ was successful in cultivating the wild 

 blueberry and other Ericacece by maintaining 

 the soil in an acid state, but this was looked 

 upon by most botanists as anomalous and ex- 

 ceptional. Because supposed oxylophytes were 

 occasionally found growing with supposed 

 "ealeicoles" — lime dwellers — eeologists in gen- 

 eral have been inclined to discredit the ex- 

 istence of any definite relation between native 

 plants and soil acidity. During the last few 

 years, however, newly developed methods of 

 interpreting and determining acidity have 

 been applied in several widely separated 

 regions — Sweden,^ Denmark,^ the northeastern 

 United States,* (and subsequently in India and 

 in England''), with the same result in all eases: 

 recognition of the great significance of the 

 acidity of the soil in controlling the growth and 

 distribution of native plants. 



The three investigators in question have 

 found independently that the active acidity of 

 a soil can be definitely determined by stirring 

 up a sample with pure water and testing the 

 extract with indicators adapted to show by 

 their color changes the hydrogen-ion concen- 

 tration. Arrhenius and the writer take theii- 

 soil samples from the roots of the plants under 

 study, while Olsen takes his at a unifonn 



1 " Experiments in Blueberry Culture," 1910, 

 United States Department of Agriculture Bureau 

 of Plant Industry Bulletin No. 193. 



- Olof Arrhenius : CEcologische Studien in den 

 StocTcholmer Schaeren. Stockholm, 1920. Ee- 

 view in Ecology, II, 223-228, 1921. 



3 Carsten Olsen : Studier over Jordbundens 

 BrintionenTconcentration og dens Betydning for 

 Vegetationen, saerlig for Plantefordelingen i 

 Naturen. Copenhagen, 1921. Abstract in Science, 

 LIV, 539-541, 1921. EngUsh edition promised. 



•* Edgar T. Wherry ; A series of papers on 



ferns, orchids, Ericacese, etc. 1916 ; also: 



' ' Soil Acidity and a Field Method for Its Meas- 

 urement. " Ecology, I, 160-173, 1920; to be pub- 

 lished in collected form in the Appendix to the 

 Smithsonian Annual Report for 1920. (In 1922). 



'< W. E. G. Atkins : ' ' Eelation of the Hydrogen- 

 ion Concentration of the Soil to Plant Distribu- 

 tion. " Nature, CVIII, 80-81, 1921. Also Sci. 

 Proc. Royal Dublin Acad., XVI, 369-413, 1922. 



depth of 10 cm. His samples are therefore 

 representative only of the plants rooting at 

 that depth, and not of the shallower or deeper 

 rooted ones. This renders his data as to some 

 plants uncertain, since most soils show a 

 marked acidity gradient, which may amount 

 to as much as 0.1 pu unit (equivalent to a fac- 

 tor of 1.25 in specific acidity) per centimeter 

 in depth. 



In a recent review, Clements*' has shown that 

 the production of acidity in bog soils is con- 

 nected with lack of aeration; but it does not 

 follow that the same is true of upland soils. 

 In the writer's experience the highest acidities 

 in them occur among rock fragments at the 

 summits of mountains, in the dry sands of pine 

 barrens, and in the most loosely packed and 

 thoroughly aerated vegetable debris. This 

 acidity is presumably due chiefly to the devel- 

 opment in the soil of such acids as acetic, citric, 

 and lactic, which, like their production for food 

 purposes, is an aerobic oxidation process. In 

 bogs, therefore, there is likely to be an increase 

 of acidity with depth, in dry-land soils a de- 

 crease. 



Before determining the acidity of a soil, 

 Arrhenius and Olsen allow the water suspen- 

 sion to stand for as much as 24 hours, and 

 then filter. The writer feels that long standing 

 of a soil in contact with excess water may 

 enable reactions, with resulting acidity changes, 

 to take place which would not occur when the 

 soil is in its natural condition, so that fairly 

 prompt testing seems preferable. Moreover, 

 filtration removes fine material which may well 

 contribute to the eflieet of the soil on a plant, 

 and therefore should be allowed to affect the 

 indicators also. Arrhenius and Olsen make 

 their determinations of the acidity of the mod- 

 ified and purified soil extracts with great pre- 

 cision, using a comparator, the former recom- 

 mending, however, that a method of deter- 

 mination should always fit the sample. The 

 writer, finding that the variation from one 

 root to another of a single plant, or from one 

 individual to another of the same species, often 

 amounts to 0.5 Pn unit, or a factor of 3 in 



« ' ' Aeration and Air-content ; the Eole of Oxy- 

 gen in Root Activity." Carnegie Institution of 

 Washington Publ, 315, 183 pp., 1921. 



