Hoagland —28— Plant Nutrition 



essential elements four more — boron, manganese, cop- 

 per and zinc.* 



So many investigators have now found it possible 

 to demonstrate boron and manganese requirements for 

 so many species of higher plants that these two ele- 

 ments are accepted, apparently, by all investigators. 

 Some statements in the literature imply that not all 

 are entirely ready to accept copper and zinc as gen- 

 erally indispensable elements. Yet in our laboratory 

 we have never failed to show a need for these ele- 

 ments by any of the many species of plants subjected 

 to rigorous test. It is well to recall that copper or 

 zinc may appear unessential merely because enough 

 of these elements gain entrance to the culture solution 

 through impurities in reagents, in culture vessels or 

 in the distilled water. Thus it is easy to fail to show 

 a need for copper if ordinary laboratory distilled 

 water is used in making the culture solution. (See 

 plates 8 and 9). 



Recent research in California by Arnon and Stout 

 (1939) gives strong evidence that another element will 

 have to be added to the list of essential elements, name- 

 ly, molybdenum. The need of this element by certain 

 fungi had already been established. The list of essen- 

 tial elements is not closed. One can say about almost 

 any chemical element that appears not to be essential 

 only that it is not required in greater quantity than is 

 represented by the unavoidable impurities in the culture 

 solution. While not conclusively demonstrated to be 

 essential over a wide range of species there are some 



* Elements in this category have been called by students of 

 plant nutrition "rare" elements, "minor" elements or "trace" 

 elements. All these terms seem to be inappropriate and I 

 prefer as a general term "micronutrient" elements, fully realiz- 

 ing that objection can be found to this term also. Iron might 

 logically be classified as a micronutrient element, but this is 

 usually not done for the reason that this element has so long 

 been recognized as a member of the list of essential elements 

 along with those needed in larger quantities. A well illustrated 

 book, "Hunger Signs in Crops", gives many examples of symp- 

 toms of deficiencies of these elements. 



