EXCHANGE OF IONS 
57 
roots may function efficiently in dilute solutions provided the con- 
centration of no single ion is too greatly reduced. We seem to find 
here an argument for Liebig's "Law of the Minimum." When a 
sufficient concentration of an ion is present, the particular ratio of 
the different ions to one another is, within a rather wide range of 
variation, relatively immaterial. The significance of a full quota of 
anions as well as of cations stands out as the most striking feature of 
the second experiment. 
On account of the different and varying temperature conditions 
under which the two experiments were carried out it would be unsafe 
to compare them too minutely. There can be no doubt however 
about the significance of the great difference between the absorption 
maxima in the two series. They are graphically represented in figure 
3, and show the strikingly greater absorption which resulted from the 
presence of a full quota of anions. 
Conclusions 
1. In general, seedlings of Lupinus alhus L. absorb more salts from 
mixtures of the nitrates of potassium, calcium and magnesium than 
from equally concentrated solutions containing only one or two of 
these nitrates. 
2. The solutions of the 3 nitrates which were most favorable to 
absorption were much inferior to corresponding solutions in which 
three anions, H2P04~, NOs" and S04=, were present. Under fairly 
comparable conditions the roots were able to absorb about half of 
the salts from the best solutions of the 3 nitrates and 85 percent from 
corresponding solutions with mixed anions. 
3. In solutions of KNO3, Ca(N03)2 and Mg(N03)2, as well as in 
solutions of KH2PO4, Ca(N03)2 and MgS04 the best absorption occurs 
when no single ion greatly predominates over the rest. Nevertheless, 
there is a wide range of variation in the proportion of different ions, 
within which range the roots absorb with almost equal efficiency. 
Bureau of Plant Industry, 
Washington, D. C. 
