4i8 UTILIZATION OF MINERAL SALTS 



ers on this problem were the German botanists, Sachs and Knop. Their in- 

 vestigations, conducted by the method of solution cultures (see later) and 

 subsequently confirmed by a number of other workers, indicated that in addi- 

 tion to the elements carbon, hydrogen, and oxygen, obtained by plants from 

 water or from atmospheric gases, the only essential elements were nitrogen, 

 phosphorus, sulfur, calcium, magnesium, potassium, and iron, all of which 

 enter the plant from the soil. 



From the work of these investigators and others developed the almost 

 classical precept that ten elements, and ten only, were essential for the ex- 

 istence of green plants. This viewpoint was first seriously challenged by 

 Maze (191 5) who considered that at least several other elements are essen- 

 tial for the continued development of green plants. The older concept of 

 the "ten essential elements" was so generally accepted, however, that Maze's 

 contentions evoked very little immediate interest. Only in comparatively 

 recent years have extensive studies been undertaken on the problem of the 

 possible roles of other elements in plant metabolism. 



It is now realized, however, that there were certain unrecognized sources of 

 error in the experiments upon which the conclusions of earlier investigators 

 were based. Almost all of their investigations were pursued by the method 

 of solution cultures, in which "pure" chemicals in certain proportions were 

 dissolved in distilled water, and these solutions were used as the medium in 

 which the plants were rooted. Many of the "pure" chemicals used, how- 

 ever, contained at least traces of other compounds which might be sufficient in 

 amount to supply plants with an adequate quota of certain necessary ele- 

 ments, especially if they were required only in minute quantities. Similarly, 

 the elements stored in the seed were not usually considered in such experi- 

 ments. The amounts of some elements available to a plant from this source 

 might suffice for its entire life history if they were only required in small 

 quantities. Furthermore, it has also been more generally realized in recent 

 years that small amounts of certain elements often dissolve in solution cul- 

 tures from the walls of the containers and thus become available for utiliza- 

 tion by plants. Traces of silicon and zinc, for example, may dissolve out of 

 the walls of ordinary glass vessels into solutions contained within them. Even 

 distilled water, of the grade generally used in such experiments, may contain 

 amounts of elements required only in traces sufficient to supply the needs of 

 the plants. For these reasons, therefore, it is clear that in practically all of 

 the earlier experiments designed to determine which elements are essential for 

 plants, small quantities of various elements other than those deliberately sup- 

 plied were usually present in the solution cultures. The failure of earlier in- 

 vestigators to recognize the possibility of the presence of such contaminating 



