Rothamsted Symposium on Trace Elements 6 



nutrient elements; quick chemical tests on plant tissues; chemical 

 analysis of plants and plant parts (especially leaves); soil tests; 

 field trials with fertilisers and special indicator plants. 



To those of you who are interested in the details of these 

 methods, I would refer you to my book "The Diagnosis of Mineral 

 Deficiencies in Plants" (1). 



The Visual Method, Its Basis and Use:— Before proceeding 

 to discuss the application of the visual method to problems of trace 

 elements in plants it may be useful to refer to the basis of this 

 method and to the way in which it is applied to nutritional prob- 

 lems. 



The visual method has as its basis the fact that when plants 

 are deficient in any one of the essential mineral nutrients or when 

 elements are present in excess they develop well defined and often 

 characteristic symptoms of the deficiency or excesses. The effects 

 produced are generally most clearly seen in the leaves, but they 

 may also show in shoots, stems, flowers, fruits and roots. Thus 

 leaves are usually particularly important in applying the visual 

 method, but all parts of plants may be used, according to the par- 

 ticular deficiency or excess. 



The symptoms produced allow of some grouping and classifica- 

 tion but to use the method extensively a large amount of detailed 

 knowledge for particular crops is necessary. 



The symptoms are established by growing plants in solution 

 cultures or in sand under stricdy controlled conditions of nutrition 

 and are later checked, where possible, under field conditions by 

 means of special fertiliser plots and by instances occurring in 

 general farm practice. In making these checks the other comple- 

 mentary methods are generally included. The visual method 

 may be applied both to intensive laboratory studies in plant physi- 

 ology and pathology involving mineral nutrition, and to practical 

 field problems of crop nutrition. Examples of the former uses 

 will be given later whilst a detailed description of the application 

 of the method in the field is given in my book. 



The Application of the Visual Method to Trace Element 

 Problems:— The manner in which the visual method has been 

 applied in our investigations to trace element problems in plants 

 may be discussed under four headings: 



a) Deficiencies. 



