A'i'KiNS — Factors affecting Hydrogen Ion Concentration of Soil. 393 



21ie relation of the hydrogen ion coneentration of the soil to plant distrihition. 



Wherry (1916) investigated the distribution of "the walking fern," 

 Camptosorus rhisophi/llus, iu relation to the content of calcium salts in the 

 soil. This was reputed to be a calcicolous species. He was unable to find 

 any correlation, and sought for an explanation iu terms of the hydrogen ion 

 concentration. The habitats examined, however, gave a very extended range 

 of values, from pH 5-5-9-0. In his further papers, which are of great 

 interest, he traced the limits for numerous species of native orchids, for the 

 Ericaceae and associated plants, and showed how it comes about that plants 

 typical of the New Jersey Pine Barrens are found bordering a salt marsh. 

 The distribution of species of ferns was also studied by Wherry, who pointed 

 out the bearing that these researches have upon horticultural problems. He 

 thinks it possible to work to an accuracy of pH 0'2, or in the field to pH 0'5. 

 This, however, is considered entirely adequate, for from one plant to another, 

 or even from one root to another, on the same individual obsei'vations may 

 differ by pH 1. The writer would, however, point out that Wherry probably 

 means this to hold only over certain ranges of reaction, for in his own 

 experience in acid soils of a particular region great uniformity, to pH 0-2 or 

 less, may be met with. The same is true of the more alkaline soils. But 

 with slightly buffered soils near neutrality varying amounts of carbon 

 dioxide have a relatively large effect. 



Some plants appear to have narrow limits of growth, such as certain 

 orchids and heath plants, with which special fungi aie associated, as 

 mycorhiza. Again, leguminous plants with root nodules are in their distribu- 

 tion possibly limited by the pH values the bacteria can endure in the soil. 



The species studied by Wherry in the U.S.A. are for the most part 

 absent from the British Isles, and the difference in climate may also to some 

 small extent alter the reactions of various situations, so it is of interest to 

 study the problem afresh. 



An elaborate contribution to physiological ecology has also been made by 

 Arrhenius (1920), who records not only pH values for different plant 

 associations, but also moisture content, organic matter, calcium, potassium, 

 total nitrogen, nitrate and ammonium nitrogen, and phosphate. The original 

 should be consulted for its valuable details. 



In the account which follows, the pH values given for a plant are either 

 from separate districts, or from a quite different soil or situation in the same 

 district. In the field one cannot fail to be impressed with the absence of 

 certain plants from what would appear to be suitable situations ; an explana- 

 tion is often found when the reaction of the soil is examined. As a general 



