THE EXCHANGE OF IONS 
3, In mixtures of Ca(N03)2 and Mg(N03)2 the greater absorption 
of the Mg ion in comparison with the K ion appears in the greater 
proportion of Mg to Ca seen in the most favorable ratio. The greatest 
absorption is found in the ratios 2 Ca+"'" : 2 Mg++ or i Ca++ : 3 Mg++ 
in the concentrations here tested. The great significance of even a 
small proportion of Ca is seen in the relatively high absorption made 
in a mixture containing i Ca to 9 Mg at the highest concentrations 
here tested. 
4. In pure solutions the Mg ion is much more favorable to absorp- 
tion than the K ion in the weaker concentrations while the K ion is 
more favorable in the highest concentration. They are absorbed to 
about the same extent at a concentration of 360 N X io~^ Absorp- 
tion from mixtures exceeds that from either pure solution except in 
the weakest concentrations. Although a most favorable ratio can 
hardly be designated, the general tendency of the results seems to 
indicate that a high proportion of Mg is more favorable in weaker 
concentrations. 
Office of Plant Physiological and 
Fermentation Investigations, 
Bureau of Plant Industry. 
