74 THE SOIL SOLUTION 



plants, it is generally held that sodium can not replace potassium 

 in the processes of plant growth, although Wheeler and his 

 colleagues have advanced evidence to show that a partial replace- 

 ment is possible. 1 It seems evident, however, that no gen- 

 eralizations can hold concerning the effect of the concentration 

 of any one base on plant growth which do not include recog- 

 nition of possible modifications due to the presence of other 

 bases; and the formulation of such generalizations must needs 

 wait upon a more thorough knowledge of the parts played by 

 the several mineral nutrients in the metabolism of different 

 classes of plants. 



As to forms or chemical combinations in which the inorganic 

 constituents of the soil solution are best adapted to plant growth, 

 but little can yet be said other than that the different combi- 

 nations do have an importance. Some empirical information is 

 available, such as for instance, that potassium sulphate or car- 

 bonate is a better fertilizer for some crops than is potassium 

 chloride. It is known that the mineral nutrients in the plant are 

 partly in inorganic combinations but largely in organic combi- 

 nations. But the causal relationships are yet to be worked out. 

 And finally, although some meagre experimental data have been 

 obtained as to the effect of certain inorganic constituents on the 

 absorption of others, by particular plants, the mechanism of 

 absorption itself, including the selective powers of the plant, is 

 yet wanting an adequate explanation. 



*The effect of the addition of sodium to deficient amounts of potas- 

 sium, upon the growth of plants in both water and sand culture, by B. L. 

 Hartwell, H. J. Wheeler and F. R. Pember, Report Rhode Island Agri- 

 cultural Experiment Station, 1906-7, pp. 299-357. 



