Atkins — Factors q^ecting Hydrogen Ion Concentration of Soil. 391 



Immediately to the uoith of the Stadclon Grits lie the Mid-Devonian 

 Slates, followed by the Mid-Devonian Limestone. The soil yielded by the 

 limestone varies from near neutrality to an alkalinity of pH 9'0 in quarries 

 and rocky subsoil. Thus it is possible to find soil of about pH 7'6 in both 

 Staddon Grit and the limestone formation. In the Mid-Devonian Slates 

 the soil near the northern limit, viz., next the limestone, is alkaline, pH 8'4j 

 whereas the lower beds give acid soil, pH 6'0-6'8i 



Here and there in most of this region there are small patches of Devoniaii 

 volcanic rocks, which in places are schalstein. This calcareous rock appears 

 to give an alkaline soil also. To the north of the limestone lies a great 

 expanse of Upper Devonian Slate, dotted with volcanic rocks. These slaty 

 rocks yield a soil which, like the Dartmouth Slates may be on either side of 

 neutrality, according to situation and exposure. Thus, whilst there is never 

 enough calcium carbonate to bring the alkalinity up to that of limestone 

 soils, on an average, there appears to be enough, in parts, to prevent the 

 development of any considerable acidity. The Compton district is marked 

 in the map as being schalstein and tuff, lience its greater alkalinity than 

 that of Egg Buekland, where the Upper Devonian Slates are developed. This 

 formation extends northwards for a number of miles, until the metamorphic 

 rocks adjacent to the granite appear. A stream flowing from Compton, 

 below Egg Buekland, is at about pH 8'0, pointing to the existence of 

 calcareous rock, whereas the Dartmoor water is at about pH 6'8. 



Northwards again lies Dartmoor, a granite rock covered by peaty soil and 

 siliceous debris. Here, as far as the writer's examination has proceeded, the 

 soil is acid, mostly pH 5'4 to 5'1. It is particularly interesting to note how 

 this acidity extends deep into the subsoil among tlie debris of the rock. 

 The most acid soil found was that in the crevices of a loosely piled granite 

 wall, which must have been formed almost exclusively of plant remains. 

 This soil at pH 4'1 falls in Wherry's nomenclature into the mediacid group, 

 the general reaction being subacid. 



Thus, as one passes inland from the coast to Dartmoor, a very varied 

 succession of soil reactions is encountered. It must not be thought, however, 

 that a district is of uniform reaction; one can only say that it is predominantly 

 of one reaction, as will be seen when considering the relation of the hydrogen 

 ion concentration to plant distribution. For example, Wherry (1920) found 

 that Sarracenia sp]p. abounded in ihe Dover swamps of Warren County, where 

 the reaction was acid, but in a neighbouring swamp. Green Pond, the water 

 was alkaline, and Sarracenia, though common, was only found on hummocks 

 of plant remains, mosses, &c., in which the reaction was acid. 



In contradistinction to the acidity developed on granite and felsite stands 

 the alkalinity found in sand dunes and shingle near the sea. Here the 



