Rothamsted Symposium on Trace Elements viii 



where"'"' have indicated the world-wide interest in, and importance of, 

 trace elements in food production, and the records of these shoiild provide 

 useful data for further studies of scientific and practical prohlems. 



In the past, investigations of trace element problems have been mainly 

 of two kinds: those carried out to throw light on crop failures and others 

 undertaken to ascertain the effects on plant growth of elements other than 

 those generally recognised as plant nutrients. Arising from the former, a 

 vast body of practical data relating to the effects of trace elements on crops 

 has been obtained but relatively little information of the roles of these in 

 plant growth has resvdted, whilst in the latter studies progress has been 

 slow because of the difficult nature of the problems involved. At the 

 present time, it is possible to recognise by various diagnostic methods the 

 main practical problems arising from trace elements in field crops and to 

 provide efficient remedies for most pathological conditions associated with 

 them (methods for controlling iron deficiency, however, are still unsatis- 

 factory). The great need is thus for fundamental studies and for these it 

 will be necessary to develop highly refined methods, as for example, to 

 determine whether other elements must be addded to the present list of 

 essential trace elements. Difficult problems in biochemistry also await solu- 

 tion and, in particular, it may be expected that the intensification of re- 

 search on the r6le of trace elements in enzyme systems will yield a rich 

 harvest of knowledge. 



In view of the above statements, it is not surprising that the main topics 

 discussed at Rothamsted referred to methods and to practical problems of 

 trace elements in crop production. 



The present volume records the papers presented at the conference and 

 some of the main points arising from these that were discussed. 



It was appropriate that Professor Nemec, in his opening address, should 

 have touched on some of the historical aspects of trace elements research 

 and, in particular, made reference to the pioneer water culture experiments 

 of Sachs and Stoeckard in Saxony, from which have developed the elabo- 

 rate techniques in use today. He also referred to the large number of 

 mineral elements that may occur in plants and to the difficulties this raises 

 in fundamental studies. In discussing 'accumulator' plants he illustrated 

 his remarks by exhibiting a sample of gold obtained from the ash of 

 Equisetutn -patustre from volcanic soils in Bohemia and also referred to 

 the concentration of droplets of metallic mercury in the seed capsules of 

 Holosteum umhellatutn on some soils. 



Three papers dealt primarily with methods used in the diagnosis of the 

 mineral status of plants and in the growing of plants in sand and solution 

 cultures for fundamental studies. The writer outlined the main featmes 

 of the visual diagnosis method and illustrated its application to some prob- 

 lems of plant nutrition. E. J. Hewitt described his sand culture methods 

 for large-scale investigations, including the highly refined methods used in 

 experiments on molybdenum deficiency. Subsequent to the conference 

 these methods have been further developed and techniques are now avail- 

 able for all the trace elements. D. I. Arnon drew attention to the experi- 

 mental difficulties in proving the essentiality of trace elements in plant 

 nutrition, and illustrated his points from the work of Stout and Arnon on 

 molybdenum. Two important points that arose from this discussion were 

 the formulation of criteria of essentiality of mineral nutrients and the pro- 



