38 



although the limit of endurance in sodium carbonate plus magnesium 

 carbonate is 0.01 normal, while for sodium sulphate plus magnesium 

 carbonate it is 0.03 normal, the proportion of individual seedlings 

 whose roots survive in good condition is decidedly greater in tlie latter 

 mixture than in the former. 



None of the readilj^ soluble salts occurs abundantlj^ in alkali soils 

 save in the presence of calcium sulphate or calcium carbonate, and 

 most often in contact witli both. Hence it follows that the limits of 

 endurance for the more soluble salts in the presence of these two salts 

 of calcium, as obtained b}' means of water-culture experiments, 

 should agree closely with the limits deterinined by soil investigators. 

 This i)roved to be the case, due allowance being made for the influ- 

 ence of the physical properties of a soil as compared with an aqueous 

 solution. 



, The following table serves to bring together, for read}^ comparison, 

 the limit of endurance for roots of both white lupine and alfalfa in 

 solutions of six of the easily soluble salts to which a solid excess of 

 both calcium sulphate and calcium carbonate was added, the mixtures 

 being brought to equilibrium before using. 



Table X. — Results ivith mixtures containijig two calcium salts. 





Limits for lupine 

 (Lupinus albus). 



Limits for alfalfa 

 (Medicago sativa).a 



Name of salt. 



Parts per 

 100, 000 of 

 solution. 



Normal 

 solution. 



Parts per 

 100, 000 of 

 solution. 



Normal 

 solution. 



Magnesium sulphate 



2,240 



960 



156 



2,160 



1,160 



417 



0.4 

 2 



:o3 



.3 

 .2 

 .05 



1,960 



960 



104 



2,160 



1,160 



667 



0.35 



Magnesium cliloride 



.2 



Sodium carbonate . ... 



.02 



Sodium sulphate 



.3 



Sodium chloride 



.2 



Sodium bicarbonate 



08 







a In the case of alfalfa a few roots barely survive in O.rs and in 0.3 normal magnesium sulphate, 

 while in 0.25 normal they make a noteworthy amount of growth during forty-eight hours. In 

 0.2 normal sodium siilphate they make a decidedly better growth, and in 0.1 normal sodium 

 chloride two and one-half times as much growth as in the water control. 



The close correspondence between the white lupine and alfalfa in 

 their resistance to the effects of these mixed solutions is worthy of 

 note, especially as alfalfa appears to be more sensitive than the lupine 

 to pure solutions. The onl}^ serious discrepanc}^ occurs in the mix- 

 ture of sodium bicarbonate, calcium sulphate, and calcium carbonate, 

 to which alfalfa roots appear to be nearly twice as resistant as are 

 those of white lupine. 



That in the neutralizing effect upon more toxic salts which these 

 two relatively insoluble salts exert calcium sulphate plays a much 

 more important part than does calcium carbonate is obvious from a 

 comparison of the limits of endurance in solutions to which either 

 calcium sulphate or calcium carbonate alone has been added. Indeed, 

 in the case of magnesium sulphate and of sodium sulphate the limit 

 of endurance is decidedly lower lu the presence of both calcium salts 



