320 Mineral Nutrition of Plants 



was strong that in addition to the seven elements on the classical list, 

 four others derived from soil— boron, zinc, copper, and manganese- 

 are to be included among the nutrients essential for the growth of 

 higher plants. There was a feeling of uncertainty, however, whether 

 the list was complete even with the addition of these four elements. 

 The very circumstances of their previous obscurity and newly-acquired 

 importance raised the question whether other impurities are not like- 

 wise being overlooked. These considerations have moved D. R. Hoag- 

 land to prepare several so-called A-Z solutions, containing a rather large 

 number of elements in minute quantities which were to be used in 

 supplementing the standard nutrient solutions. In an experiment with 

 strawberries, Hoagland and Snyder (23) gained the distinct impression 

 that plants receiving an enlarged A-Z solution, containing 22 elements 

 in addition to the four micronutrients, were superior to all others. This 

 observation, although strongly suggestive, did not lend itself to quanti- 

 tative evaluation. Soon afterwards in an investigation of the relative 

 merits of ammonium and nitrate as sources of nitrogen, it was found 

 that molybdenum, chromium, or nickel improved the growth of barley 

 plants in a culture solution supplied with ammonium salts as the sole 

 source of nitrogen (/). 



These observations seemed to justify the undertaking of a systematic 

 investigation to test the hypothesis that the list of micronutrients, then 

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

 was done by arranging a number of elements in groups and by observ- 

 ing how the addition of a given group affected the growth of plants in 

 culture solutions (2). 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 solution, 

 designated B7, contained the following seven elements: molybdenum, 

 vanadium, chromium, nickel, cobalt, tungsten, and titanium— a some- 

 what arbitrary grouping based on the consideration that each of these 

 could assume various valency levels and hence, conceivably, participate 

 in oxidation-reduction processes within the plant cell. The already 



