28 



pleted, the data thus far obtained throw so much light on the whole 

 subject of alkali soils, and go so far to account for the fact that the 

 limits of endurance of plants in pure solutions of the various salts 

 are low as compared with those determined from the observations 

 of survey parties in the field, that it seems advisable to present 

 them here. 



In the case of mixtures of two readily soluble salts, solutions of 

 each, of twice the desired concentration, were mixed in equal vol- 

 umes. Where one of the salts is a comparatively insoluble one, it 

 was added in solid form to a solution of definite concentration of the 

 soluble one, and the mixture was then diluted to the required con- 

 centration, as though the more soluble salt alone were present. (The 

 source of error incurred by this method was considered so slight as 

 to be practically negligible.) The mixture was then allowed to 

 stand for a week or ten days with frequent shaking, in order to bring 

 it to equilibrium before using. In all mixtures of magnesium car- 

 bonate and of calcium carbonate alone with other salts, the undis- 

 solved residue was removed by filtration. Likewise in earlier experi- 

 ments with calcium sulphate added to other salts, the residue was 

 removed, but in those upon which are based the limits given in the 

 tables it was retained. In all cases where both calcium sulphate and 

 calcium carbonate were added, the undissolved residue remained 

 during the culture. The difference in limit due to the presence or 

 absence of a solid excess was, however, usually imperceptible, and 

 always slight. 



In every c^se the object was to ascertain how far the limit of endur- 

 ance for the roots in the presence of the more toxic salt could be 

 raised by addition of one that is less injurious. Although the con- 

 centration of solution of the latter is invariably stated, if it be a 

 readily soluble salt, it is the concentration of solution of the more 

 poisonous salt as denoting a corresponding limit of endurance to 

 which attention is chiefly directed. It is interesting that in cases 

 where hoth of the salts mixed are readily soluble ones the less toxic 

 salt appears usually to be more effective in neutralizing the more 

 toxic one when added in concentration somewhat above rather than 

 below that in which plant roots will endure it when alone. Thus, in a 

 mixture containing equal volumes of 0.0075 normal sodium carbonate 

 and 0.01 normal sodium sulphate, roots of two plants survived, but all 

 died when the mixture contained 0.0075 normal sodium carbonate and 

 only 0.005 normal sodium sulphate. Also a majority of the roots 

 could retain their vitality in a mixture containing equal volumes 

 of 0.0025 normal magnesium sulphate and of 0.01 normal sodium 

 sulphate, but not in 0.0025 normal magnesium sulphate plus 0.005 

 normal sodium sulphate. Similar results were obtained by adding 

 sodium sulphate to magnesium chloride and sodium chloride to mag- 

 nesium sulphate. The reverse was true, however, in the mixtures of 

 magnesium chloride and sodium chloride, the less concentrated solu- 

 tion of the latter proving more beneficial. 



