314 Mineral Nutrition of Plants 



then the question, what specific elements are essential in plant nu- 

 trition, it is necessary to determine which of the elements derived 

 from the soil are required for plant growth through the stage of 

 reproduction. The simplest approach to this problem is an analytical 

 one. An accurate analysis of the inorganic constituents of plants 

 should give an inventory of the elements derived from the soil. This 

 is indeed the case, but the list of elements derived from the soil, un- 

 fortunately, is not synonymous with the list of elements essential 

 for plant life (22). The plant has the capacity, within rather wide 

 limits, of indiscriminate absorption: it absorbs essential as well as 

 superfluous or even harmful elements. Every essential element must 

 of course be present in the plant, but not every element present is 

 essential. It will be shown later how a failure to recognize this funda- 

 mental property of plants may lead to erroneous conclusions drawn 

 from otherwise convincing evidence. 



Since the composition of the plant offers no reliable guide as to 

 the essentiality of the constituent elements, we find it more profitable 

 to turn to the external medium in which the plant grows. An obvious 

 approach is to remove from the external medium, one at a time, its 

 constituent elements and to observe what effect this has on the plant. 

 If the plant failed to grow as a consequence of removing element A 

 but not element B, the conclusion would be drawn that element A 

 is essential and element B is dispensable. This straightforward ap- 

 proach, however, is not applicable to the natural soil medium in 

 which plants grow. To attempt a complete removal of an element 

 from a soil would entail for most nutrients, a chemical treatment 

 which would alter the soil from a natural to an artificial medium. 

 Nevertheless, there are soils in nature which, for one reason or an- 

 other, are deficient in plant nutrients. The addition of the deficient 

 element would restore or enhance plant growth and thus provide a 

 clue to essentiality. 



Although it is common experience in agricultural practice to cor- 

 rect nutritional deficiencies by soil fertilization, the ubiquitous com- 

 plexity of heterogenous soil components often complicates the inter- 

 pretation of observed plant responses. The difficulty arises in attempt- 

 ing to distinguish between a direct and an indirect, effect of a given 



