412 Scientific Proceedings, Royal Dublin Society. 



In conclusion, it must be stated that the experimental work recorded in 

 this and the accompanying papers was carried out in the School of Botany, 

 Trinity College, Dublin, and at the Marine Biological Laboratory, Plymouth. 

 Determinations were also made at the Agricultural Eesearch Institute, Pusa, 

 India, and at various other places during field work. To the Directors of the 

 laboratories mentioned the author wishes to record his indebtedness for the 

 facilities aftbrded. The work was rendered possible by a grant from the 

 Department of Scientific and Industrial Eesearch, which covered the 

 expenses of the special reagents. Thanks are also due to Mr. W. A. Davis, 

 Pusa, for numerous samples of soil; to Dr. E. L. Fox, Plymouth, and Mr. 

 C. A . Pode, Slade, for water samples and access to streams ; also to Miss M. V. 

 Lebour for some plankton identifications. 



The bibliography is common to the three papers issued in this cover, and, 

 with but few exceptions, references to be found in Clark's (1920) list have 

 been omitted from it. They may be identified there by name and date given 

 in the text. 



Summary. 



1. The theoretical maximum alkalinity due to calcium carbonate only is 

 pH 9'01, which may be attained experimentally in the absence of carbon 

 dioxide. The corresponding bicarbonate in equilibrium with the gases of the 

 atmosphere is at pH 8'37 at 16° C, becoming more alkaline at higher 

 temperatures. Owing to the high content of carbon dioxide in the soil, the 

 pH values of limestone soils are usually lower, and vary with the aeration. 



2. The theoretical maximum alkalinity for magnesium carbonate is 

 pH lO'O. Dolomite soils may thus attain to greater alkalinity than limestone 

 soils. 



3. Alkalinity of over pH 10, due to sodium carbonate, may be reduced 

 to pH 8 by the addition of calcium sulphate. The former reaction is 

 injurious or destructive to vegetable cells, whilst the latter is favourable to 

 most plants. 



4. Soil acidity may be occasioned by the oxidation of sulphur from iron 

 pyrites. This acidity favours the production of available phosphate, and is 

 accordingly beneficial to certain plants. 



5. Owing to production of carbonic acid by bacteria, a soil extract may 

 decrease in alkalinity from pH 8-7 to 7'2, or less. The result in the soil 

 appears to be to render iron salts more readily available in calcareous soil 

 when inundated than when uncovered. The alteration is usually more rapid 

 in soils from the top six inches than at greater depths. 



