33 Arnon: Criteria of Essentiality 



growth experiments intended to determine the essential status of 

 a given element. Because of the complexity of the plant-soil sys- 

 tem it is inescapable that such experiments will be carried out in 

 artificial culture media. In this undertaking, the formulation of 

 definite criteria of essentiality can be of distinct aid. In our lab- 

 oratory this subject was linked with the discovery of the indis- 

 pensability of molybdenum for the growth of higher plants (2). 

 A recounting of this investigation will serve to illustrate our pres- 

 ent point of view. 



In a comparison of ammonium with nitrate as sources of nitro- 

 gen, a preliminary experiment indicated that molybdenum, chro- 

 mium and nickel improved the growth of barley plants in a cul- 

 ture solution supplied with ammonium salts as the sole source of 

 nitrogen (3). This suggested that the list of micronutrients, then 

 confined to boron, manganese, zinc and copper was incomplete. 

 This possibility was tested by arranging a number of elements 

 in groups and by observing how the addition of a given group 

 affected the growth of plants in culture solution (4). Three 

 supplementary solutions, each containing different elements in 

 minute quantity, were prepared. One solution, designated A4, 

 furnished the recognized four micronutrients, boron, manganese, 

 copper and zinc. The basic culture solution, supplemented with 

 the A4 solution, therefore furnished the plant with a seemingly 

 complete list of essential elements. Another supplementary solu- 

 tion, designated B7, contained the following seven elements: 

 molybdenum, vanadium, chromium, nickel, cobalt, tungsten and 

 titanium— a somewhat arbitrary grouping based on the considera- 

 tion that each of these could assume various valency levels and 

 hence, conceivably, participate in oxidation-reduction processes 

 within the plant cell. The already mentioned findings on the 

 role of metals in the nitrogen nutrition of barley (3) suggested 

 this particular grouping. The third supplementary solution, des- 

 ignated Cl3, supplied thirteen elements: aluminum, arsenic, cad- 

 mium, strontium, mercury, lead, lithium, rubidium, bromine, io- 

 dine, fluorine, selenium and beryllium. Sodium and chlorine, 

 though not singled out, were provided from several sources in 

 these solutions. 



In experiments with lettuce and asparagus (4) a marked im- 

 provement in growth was observed from supplying, in addition to 

 A4, the B7 solution. The further addition of thirteen more ele- 

 ments supplied by the CI 3 solution produced no measurable effect 



