378 Scientific Proceedings, Royal Dublin Society. 



biological decomposition, the accumulation of carbohydrates and similar 

 organic matter is favoured by an acid reaction, and in alkaline soils, such as 

 those of Bihar, India, organic matter may become reduced to such low 

 amounts as to lessen crop production (Davis, 1920). 



As mentioned further on, changes in soil reaction induced by manuring 

 may be appropriately studied by the indicator method. 



The relation of the ions of the soil solution to its hydrojcn ion content. 



When studying soil reaction it is well to consider the maximum possible 

 effects produced by the ions present, considered salt by salt. Most attention 

 in this country and the eastern states of U.S.A. has been given to soil 

 acidity, but in some western states, in India, Egypt, South Africa, and 

 elsewhere, alkalinity is the bugbear. The question has been discussed in a 

 recent monograph by Harris (1920). In popular language, "alkali soil" 

 denotes both soil sterile from abundance of neutral salts, which is termed 

 " white alkali," and the truly alkaline " black alkali " soil, in which sodium 

 carbonate is present. The action of the latter on the organic matter gives 

 rise to the black colour. White alkali soil may be reclaimed by irrigation 

 with efficient drainage, whereas black alkali soil is far less tractable, since the 

 alkalinity deflocculates tlie clay particles. 



(a) Calcium salts. — Consider now the reaction of water in contact with 

 pure calcium carbonate, such as calcite crystals. This may be taken as the 

 first stage in the production of an alkaline reaction. In the — hypothetical 

 in nature — absence of carbonic acid, the salt has a very slight solubility, 

 which, according to Euppin (1909), quoting Schloesing, is at 16° C. 0*0131 

 grams per litre, and has a hydroxyl number of 1'05 x 10'^ namely, that 

 number of gram equivalents per litre. This corresponds to pH 9"015. 

 Accordingly this limit cannot be surpassed in the more alkaline direction by 

 a solution of pure calcium carbonate In order to test this directly, pure 

 calcite crystals were boiled with water both immediately and after standing 

 for some days. As tested colorimetrically by the method of Clark and Lubs, 

 pH 9'0 was only very slightly surpassed by the saturated solution. With 

 limestone, which may contain small amounts of magnesium, pH 9*2 was 

 reached. The standards are at intervals of pH 0'2, but interpolations may 

 be made. With clear solutions and indicators which show an intensity 

 change only, namely, not a change in tint, or a negligible one, precision can 

 be attained by the use of a Duboscq or Kober colorimeter. The depth of 

 colour varies with the hydrogen ion concentration, but the pH scale is the 

 negative logarithm of this. Over an interval of pH 0*2 it has been found 



