3 2 4 Mineral Nutrition of Plants 



tion of molybdenum to molybdenum-deficient cultures was accom- 

 panied by the other elements in the B7 and C13 solutions. 



To test the results by the last criterion, that of the direct effect of 

 an element on the plant as distinguished from its possible influence on 

 the root environment, molybdenum-deficient plants were sprayed with 

 a dilute solution of molybdic acid (1 p.p.m. molybdenum) so as to 

 bring about absorption only through the aerial parts of the plant. Re- 

 covery and resumption of normal growth with the disappearance of 

 the molybdenum-deficiency symptoms took place. This provided the 

 last link in the chain of evidence for the indispensability of molybdenum 

 for the tomato plant. 



The results with the tomato plant were subsequently repeated with 

 mustard and lettuce. The essentiality of molybdenum was soon con- 

 firmed by Piper (ja), Hoagland (2/), and more recently by Hewitt 

 and Jones (19) and Mulder (jS). Molybdenum was recognized as an 

 essential element and included with the A4 solution which was re- 

 designated A5, whereas the B7 was changed to B6. The addition of A5 

 to a basic nutrient solution gave for the species tested the same results 

 as a further addition of B6 and C13. 



Does the inclusion of molybdenum among the essential elements 

 complete their list? An unequivocal answer to this question cannot be 

 given, notwithstanding the fact that a number of different species of 

 plants have been grown successfully in rigidly purified nutrient solu- 

 tions which supplied only boron, manganese, copper, zinc, and mo- 

 lybdenum. It is certain that, despite all caution, minute impurities of 

 other elements persisted in the nutrient medium as well as in the seed. 

 It would seem best to attempt to answer this question in a quantitative 

 rather than a qualitative manner: to determine analytically whenever 

 possible the upper limit of impurity for a given element that may be 

 contained in the nutrient medium and to measure a growth response 

 with and without a further addition of the element in question. This 

 can be illustrated as follows. It was found with the dithizone test that 

 when the combined zinc, copper, lead, cadmium, and mercury content 

 of a nutrient solution was less than 0.0001 mg. per plant, severe defici- 

 ency symptoms occurred in the tomato. Recovery was brought about 

 by adding 0.002 mg. of copper and 0.002 mg. of zinc, but no further 



