WATER SOILS. 429 



necessarily ((Hitaiii sonu' bicarhonatt', and, conversely, solutions of hicarlionate nec- 

 essarily contain some normal carbonate. At equilibrium, wiiicli depends upon the 

 concentration of the scjhition, the temperature, and the jjartial pressure of the carbon 

 dioxid in tlie surroundinj; atmosphere, there is a definite distribution of the base 

 between tlie two acids. At any given concentration the i)roportion of normal car- 

 bonate increases witli rise of temperature. Conversely, at any given temperature 

 the proportion of normal carbonate increases with the total concentration, provided 

 the latter is not very great. In presence of normal or excessive amounts of carbon 

 dioxid calcium is found almost entirely in form of bicarbonate. Magnesium, how- 

 ever, under the same conditions, is largely (over 50 per cent) in form of normal car- 

 bonate. "It seems probable, therefore, that the pre.«ence of much magnesium 

 carbonate, as compared with calcium carbonate, in the soil would have a much more 

 marked effect in producing solul)le normal carbonates or the dreaded 'black alkali.' " 



Studies on the solubility of gyi)suin in aqueous solutions of sodimn chlorid (alone 

 or in the presence of solid calcium carbonate) and of certain other electi'olytes — mag- 

 nesium chlorid, calcimn chlorid, sodium sulphate — are reported, which developed 

 certain facts of value in exj)laining the solution and transportation of gypsum in the 

 soil, its precii)itation from the soil solution in the presence of large (juantities of other 

 soluble salts, and the formation of gypsum layers at different depths in the soil as 

 observed in many localities of the arid region. The light which these studies throw 

 on the formation of alkali and alkali crusts has been utilized in a comprehensive 

 classification of alkali lands on a chemical basis (E. S. R., 13, p. 232). 



" It seems probaljle, from the results of this investigation, that wherever gypsum 

 occurs in the soil thei'e will be a considerable amount of calcium in the soil solu- 

 tion. This is now known to be a matter of great importance from a ]ihysiological 

 point of view, enabling the plants to withstand nuu-h larger (juantities of the more 

 soluble salts than would otherwise be possible. . . . 



" Calcium sulphate is nuich more soluble than calcium carbonate, even when there 

 is a considerable amount of carlxjn dioxid present, and the solubility of the latter 

 salt is thus increased through the formation of the more soluble hydrogen carbonate. 

 It has therefore seemed ])rol)able that it would be better in manv cases to use the 

 sulphate for liming soils, l)ecausein this way more calcium is brought into theground 

 solution and its ratio to tlie other bases dissolved is much increased, and because 

 its active mass in the solution is greater and its effect upon other soil components is 

 corresi)ondingly increased. On the other hand, it can not have the effect that either 

 the carbonate (jr hydrate of lime has in counteracting an excess of carbonic or other 

 organic acids in sour soils or in making them slightly alkaline — a condition which 

 appears to be desirable for certain crops. Furthermore the carbonate and hydrate 

 have certain well-marked effects on the texture of soils, which are sometimes of as 

 mu(;li or moi-e importance than the purely chemical results; consequently it seems 

 desirable that the comparative value of these lime salts, or mixtvires of them, on soils 

 of various charactei' should receive more attention than has hitlierto been given, 

 both in the laboratory and in actual field practice. 



"The application of soluble mineral fertilizers to the soils of humid areas unc]ues- 

 tionably affects the solubility of mineral components of the soils. Such cases are 

 analogous to the gypsum solution, in that a slightly soluble substance is brought into 

 contact with solutions of much more soluble substances. The idea suggests itself 

 that, as with gypsum, pc^ssibly the mineral components of the soil may have maxi- 

 mum .solubilities, in solutions of the more soluble salts of definite concentrations, far 

 Ijelow the limit of solubility with respect to the readily soluble salts." 



Investigations of the same kind on the solubility of calcium carl)onate in aqueous 

 solutions of certain electrolytes in equilibrium with atmospheric air showed "that 

 sodium chlori<l in solution has an important nMe in causing the solution, transporta- 

 tion through the soil, and reprecipitation of calcium carbonate, and that this role is 



