SOIL CONDITIONS AFFECTING PLANT GROWTH 79 



but was killed in a few minutes by N NaCl, and in a few 



32 



days by *oooi N NaCl ; yet the toxic effect even of the 

 stronger solution disappeared on adding one gram-molecule 

 of CaCl 2 for every loo gram-molecules of NaCl. It does not 

 appear that calcium prevents the entrance of the sodium or 

 other ion into the plant : apparently it gives greater vigour to 

 the plant 1 Magnesium chloride and sulphate, potassium 

 chloride and calcium chloride were also toxic when used 

 singly, but in admixture they formed a nutrient medium in 

 which the plant grew normally and developed fruit even 



when N NaCl was also present. This action is called 



3 2 



antagonism of ions. 2 Osterhout shows (220^) that the phen- 

 omena hold generally both for land and water plants. 



Other facts are less easy to explain, such as Grafe and 

 Portheim's observation that the toxic effects of a single salt 

 fail to appear, or are much delayed, when sugar is supplied. 3 



These and many other experiments all indicate that a 

 complex equilibrium normally exists in the cell between 

 colloids and electrolytes which can be maintained only when 

 the external medium has an appropriate composition. 



Even when all the nutrient salts are present and the total 

 osmotic concentration is maintained constant at a suitable 

 level it is still possible to produce the most diverse effects on 

 the growing plant, from violent injury to excellent growth, 

 by varying the proportions in which the salts occur. This 

 problem has been much investigated by Tottingham (283) and 

 by Shive (262^). Tottingham studied the effects of eighty-four 

 mixtures of KH 2 PO 4 , KNO 3 , MgSO 4 and Ca(NO 3 ) 2 , plus a trace 

 of an iron salt, all of which had the same total osmotic con- 

 centration. Shive simplified the investigation by reducing the 

 salts to three, KH 2 PO 4 , Ca(NO 3 ) 2 and MgSO 4 , and using 



1 J. A. Le Clerc and J. F. Breazeale, jfourn. Ag. Research, 1920, 18, 347 ; 

 L. Maquenne and E. Demoussy, Compt. Rend., 1920, 170, 420. 



a For sand culture experiments see Wolkoff, Soil Sci., 1918, 5, 123-150. 

 9 Bied. Zentr., 1908, xxxvii., 571. 



