Special Functions and Relations 181 



In cases where iron is deficient in the soil, or held as 

 markedly insoluble compounds, beneficial results have 

 been obtained by the application of a soluble salt. Rich- 

 ards and Ono have shown that iron salts have a remarkably 

 stimulating effect upon filamentous fungi, increasing the 

 dry weight several fold over that obtained when the mini- 

 mum used is merely that which would occur as impurities 

 in the purest salts. Final proof of the relation of diverse 

 plants to iron is most difficult to obtain, owing to the pres- 

 ence of traces of this metal in many of the purest salts. 



97. Sodium. - - Sodium, a metal indispensable in ani- 

 mal nutrition, is not required by plants. It would seem 

 that it may at times prove beneficial, and in the field 

 relations of crops it is often indirectly serviceable by setting 

 free other requisite bases. Breazeale has shown, by experi- 

 ments interesting both with respect to method and result, 

 that more sodium is absorbed, and that it may be directly 

 beneficial, in the absence of sufficient potassium. 



Wheeler has conducted extensive field experiments, 

 upon various aspects of the sodium problem. This work 

 supports the views advanced, in a measure. It also indi- 

 cates that field applications of sodium may be beneficial 

 in subsequent years " in those cases where the previous 

 application of potassium salts had been large." He re- 

 gards this as " due, in part at least, to the retention in the 

 soil of a part of the previous applications of potassium 

 salts, by virtue of extra soda having been taken up by the 

 preceding crops in the place of superfluous potash, whereby 

 the potash supply in the soil was really conserved." 



98. Chlorine. - - Chlorine seems to be generally ines- 

 sential for the complete development of the higher plants. 



