14 TOLERANCE OF PLANTS FOR SALTS IN ALKALI SOILS. 



In tolerance of the two magnesium salts, maize heads the list and 

 cotton comes last. To magnesium sulphate maize is from thirty- 

 six to two hundred times as resistant as the lupine, five hundred 

 times as resistant as the beet, and eight hundred times as resistant as 

 cotton. Even among the four Graminea? tested there is great differ- 

 ence in tolerance of this salt, maize being one hundred and thirty- 

 three times as resistant as oats. Only less striking are the differences 

 in tolerance of magnesium chlorid, maize, the species that is most re- 

 sistant to this salt, enduring a concentration of solution two hundred 

 times as great as that which is critical for cotton, the least resistant 

 species. 



Less striking results are brought out in the presence of the sodium 

 salts, yet here also marked differences in tolerance occur. Maize en- 

 dures three times as much sodium carbonate, ten times as much sodium 

 sulphate, seven times as much sodium chlorid, and eight times as much 

 sodium bicarbonate as does cotton. 



It is evident from Tables III and IV and from Harter's results with 

 wheat that different plants have widely different degrees of variability 

 inside the limits of the species as regards resistance to salt solutions. 

 Thus the Red Algerian oat is three times as resistant to magnesium 

 sulphate as the Culbertson variety, and of the nine varieties of wheat 

 with which Harter experimented some were three times as tolerant of 

 magnesium chlorid and of sodium carbonate as were others. a Of four 

 varieties of Androjwgon sorghum, one, the Early Amber variety, en- 

 dured five times as great a concentration of magnesium chlorid as that 

 which was critical for the Dagdi Juar variety. Comparing two closed 

 related species of the same genus, Gossypmm harbadense and G. hir- 

 sutum, we find (Table II) that the former is twice as resistant as the 

 latter to sodium carbonate. 



RESULTS WITH MIXED SOLUTIONS. 



As we have seen, the results of the experiments with solutions con- 

 taining only a single salt can not be correlated with our knowledge of 

 'the relative resistance of the plants used when growing in natural 

 "alkali" soils. Kearney and Cameron 6 pointed out that the clue to 

 this discrepancy was to be found in the fact that in nature we have 

 always to do with a mixture of salts and never with pure solutions. 

 They found that by adding sodium salts to the solutions of magnesium 

 salts the critical concentrations of the latter could be raised consider- 

 ably and that the neutralizing effect in the case of Lupirms alhus and 

 Medicago sativa became enormous when salts of calcium were added 

 to solutions of the sulphates and chlorids of magnesium and sodium. 



"See Bui. 79, Bureau of Plant Industry, U. S. Pept. of Agriculture, pp. 25 and 27. 

 &See Report 71, U. S. Dept. of Agriculture, p. 27. 



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