18 



TOLERANCE OF PLANTS FOR SALTS IN ALKALI SOILS. 



Table X. — Minimum amounts of calcium sulphate giving the maximum neutralizing 

 effect upon the toxicity of other salts to Andropogon sorghum. 





Critical concentration 

 when saturated with 

 calcium sulphate. 



Minimum amount of 

 calcium sulphate giv- 

 ing neutralizing effect 

 equivalent to that of 

 saturation. 



Salts neutralized. 



Fractions of 

 a normal 

 solution. 



Grams per 

 liter. 



Fractions of 

 saturation 



in the total 

 volume of 



water in the 



mixed solu- 

 tion. 



Grams per 

 liter in the 



mixed 

 solution. 





0.15 

 .0225 

 ,2 



8.8 

 1.2 

 9.5 



0. 053 

 .183 

 .3 



0.112 





389 



Magnesium chlorid 



.637 



Comparing the results as to sodium chlorid given in Table X with 

 those given in Table IX, we see that, while the concentration of that 

 salt which is critical for Lupinus albus can be raised to 0.2 normal by 

 the addition of calcium sulphate, for Andropogon sorghum the limit 

 can be raised to only 0.15 normal. On the other hand, the minimum 

 amount of calcium sulphate required to produce the maximum neutral- 

 izing effect is only about one-fifth as great for the latter plant as for the 

 former. Table X also shows that the maximum neutralizing effect is 

 obtained by widely different amounts of calcium sulphate in the case 

 of each of the three more toxic salts, and that the more toxic the salt 

 in pure solution the greater is the amount of calcium sulphate required 

 to produce the maximum possible neutralizing effect. 



If we compare the limits of this variety of Andropogon sorghum in 

 pure solutions, as stated in column 5 of Table III (p. 10), with those 

 in mixed solutions, as given in Table X, we observe that the critical 

 concentration of magnesium chlorid can be raised one hundred and 

 sixty times by the addition of calcium sulphate, while that of sodium 

 carbonate can be raised onh T three and six-tenths times. Yet to obtain 

 the former result less than twice as much calcium sulphate is required 

 as to obtain the latter. 



To neutralize sodium carbonate as completely as possible, three and 

 one-half times as much calcium sulphate is required as in the case of 

 sodium chlorid, although the fraction of a normal solution representing 

 the critical concentration of the mixed solution is only one-seventh as 

 great for the former salt as for the latter. Six times as much calcium 

 sulphate is needed to produce the greatest possible neutralizing effect 

 upon magnesium chlorid as upon sodium chlorid, yet the critical con- 

 centration of the former in the mixed solution is only slightly greater 

 than that of the latter. 



That there is no close quantitative relation between the amount of 

 the more toxic salt present in the mixed solution and that of calcium 

 sulphate necessary to neutralize it is evident from the fact that, in 



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