Rothamsted Symposium on Trace Elements 34 



on either the lettuce or the asparagus plants. The results ob- 

 tained with lettuce are given in Table 3. 



Of the seven elements contained in the B7 solution only 

 molybdenum and vanadium had been associated with biological 

 phenomena in plants, especially with nitrogen fixation by Azoto- 

 hacter (5, 6). Mention was already made that under special 

 conditions of nitrogen nutrition (3) chromium and nickel were 

 associated with favourable responses on the growth of barley. It 

 was undertaken therefore to formulate definite criteria of essen- 

 tiality by means of which the status of each of the seven elements 

 comprising the B7 group could be tested. 



Table 3 (Jrom Arnon (4)): Effect of adding different groups of micronutrients 

 on the growth of lettuce plants* in culture solution. 



Microelements added Shoots Roots 



None 71.4 14.5 



A4 105.7 22.0 



A4 -I- B7 1068.3 188.6 



A4 -f B7 -f C13 984.4 196.2 



* Average fresh weight of plants in grams. 



The following criteria were set up: an element is not consid- 

 ered essential unless (a) a deficiency of it makes it impossible for 

 the plant to complete the vegetative or reproductive stage of its 

 life cycle; (b) such deficiency is specific to the element in ques- 

 tion and can be prevented or corrected only by supplying this 

 element; and (c) the element is direcdy involved in the nutrition 

 of the plant quite apart from its possible effects in correcting some 

 unfavourable microbiological or chemical condition of the soil or 

 other culture medium. 



The criterion of the foremost physiological significance is the 

 requirement of an inorganic element for the successful comple- 

 tion of the life cycle of a plant. This is, of course, different from 

 merely demonstrating a favourable effect on growth. The exper- 

 imental procedure involved in putting this criterion to the test 

 must be based on removing the element in question from the 

 nutrient medium of the plant. This, however, is beset with dif- 

 ficulties. First, it is impossible to remove completely an element 

 that may be contained in the seed. Second, the same obstacle 

 applies to the nutrient medium. Regardless of how effective pu- 



