A'i'KiNS — Factors afecting Hydrogen Ion Concentration of Soil. 395 



effects of " sourness " in soil must not therefore be thought of as due merely 

 to the acidity, which may be trifling, and far less than that of the cell-sap of 

 the roots. They are occasioned rather by a series of related causes, such as 

 accumulation of carbonic acid, possibly other acids, and of reducing agents 

 (Gillespie, 1920), deficiency of oxygen, and, in special cases, deficiency in 

 calcium salts and inhibition of the beneficial action of various soil bacteria. 

 What these conditions are for calcareous soils has recently been the subject 

 of a lengthy survey by Salisbury (1920). 



From what has been said already, it may be deduced that variations in 

 soil reaction are more likely to be found in sandstone and slate districts than 

 in those where limestone predominates. The bufier action of the carbonate- 

 bicarbonate system prevents wide divergences. The reaction may vary from 

 about pH 8-8-9-0 in a limestone quarry or near the rock to about pll 7-5 in 

 a field, or even less in a well-manured garden. A very average value is in 

 the neighbourhood of pH 8. In sea sand the comminuted shells give a 

 continuous supply of calcium salts long after much of the soluble sodium 

 salts has been washed out. Fisher (1921, 2) has drawn attention to the 

 importance of the buffer action of the soil, and instances the action of basic 

 slag in altering the reaction of a light sand, and not of a heavy loam. 



The soils of the province of Bihar, India, illustrate the uniformity of 

 reaction due to calcareous silt. Samples from various portions of the estate 

 of the Agricultural Eesearch Institute, Pusa, were at pH 8-6-9-0, and 

 identical values were given by soils from indigo estates in this province, 

 from different fields in each estate, and at depths of 0-6 in., 6-12 in., and 

 1-3 ft. Since, in addition to a number of Pusa samples, thirty-six others 

 from three estates were examined, the uniformity of the reaction is evident. 

 Obviously a study of plant distribution in such a district does not extend 

 beyond one type, the calcicolous. A good variety, however, may be obtained 

 in the Youghal district, Co. Cork, and the adjacent portions of Co. 

 Waterford. 



A road runs approximately north and south from the seashore through 

 some low-lying sandy country, which is also boggy in parts. To the east on 

 the sea-front is " Clay Castle," a sandy cliff with patches of hard clay ; to the 

 west are dunes bordering the rifle range. Further inland are fields under 

 grass, potatoes, turnips, &c., especially along a cross-road to the east. 

 Beyond the latter a glen is entered, with red sandstone quarries ; in this the 

 road rises considerably to about 150 or 200 feet above sea- level. Within this 

 limited area, bordering a distance of something over half a mile on the road, 

 the soil reaction varies from pH 8-8 to 56, and diverse types of vegetation 

 are found. 



