20 TOLERANCE OF PLANTS FOR SALTS IN ALKALI SOILS. 



not only as regards the absolute toxicity of each salt but also as 

 regards the relative order of toxicity of the salts. Of the eight spe- 

 cies used in these experiments maize (Zea mays) is, on the whole, the 

 most resistant to pure solutions, and cotton (Goss}^pium) the. least. 



(I) Seedlings grown from fresh seed are much more resistant than 

 those developed from older seed. 



(5) The presence of calcium sulphate in excess greatly diminishes 

 the toxicity of the magnesium and sodium salts to all the plants 

 tested, the neutralizing effect being greatest in the case of the sul- 

 phate of magnesium and least in that of sodium carbonate. 



(6) The addition of calcium sulphate tends to equalize the toxicity 

 of the different magnesium and sodium salts. 



(7) As a rule, the more sensitive the species to the pure solution 

 the greater is the counteracting effect of the calcium salt; hence, the 

 presence of the latter tends to diminish the differences in resistance 

 shown b} T different plant species in the presence of pure solutions. 



(8) Amounts of calcium sulphate smaller than that necessary to sat- 

 urate the mixed solution also show a marked neutralizing effect upon 

 the more toxic salt, but the minimum amount of calcium sulphate 

 capable of producing such effect remains to be determined. 



(9) For the white lupine the presence of 0.5 gram of calcium sul- 

 phate is as effective as seven times that amount in neutralizing sodium 

 chlorid, while for sorghum 0.1 gram is as effective as twenty times 

 that amount. 



(10) To secure the most effective possible neutralization of sodium 

 chlorid five times as much calcium sulphate is required in the case of 

 the white lupine as in that of sorghum, although the limits for these 

 two plants are approximately the same both in pure sodium chlorid 

 and in sodium chlorid plus an excess of calcium sulphate. 



(II) While the . comparative resistance of the different plants to 

 pure solutions of the single salts can in no way be correlated with that 

 of the same species to the different combinations of "alkali" salts 

 occurring in western soils, their behavior in mixed solutions shows a 

 much closer approach to that observed under natural conditions. 



113 



