MICRO-NUTRIENT PROBLEMS 51 



excess the bulk of it must be removed before polarographing for 

 nickel and cobalt. The author is not aware of the polarographic 

 method having been used for the determination of nickel or 

 cobalt in plant material, and Piper (1942) points out that the 

 nearness of the deposition potential of zinc to that of cobalt, and 

 the small amount of the latter relative to that of zinc, usually 

 present in plant material, renders the polarographic determina- 

 tion of cobalt in plant ash uncertain. 



The estimation of cobalt in plant material and in soil is usually 

 effected by colorimetric or absorptiometric means depending on 

 the intense coloration produced by cobalt compounds on treat- 

 ment with the sodium salt of l-nitroso-2-naphthol-3 : 6-disulphonic 

 acid, generally known as nitroso-i?-salt. Procedures have been 

 described by Kidson, Askew and Dixon (1936) and by Davidson 

 and Mitchell (1940) for the determination of cobalt in soils in 

 this way and by McNaught (1938), Kidson and Askew (1940) 

 and Marston and Dewey (1940) for the similar estimation of 

 this element in plant material. It would appear that quantities 

 of cobalt down to about 0-5^,g. are determinable in this way. 



3. The Diagnosis of Mineral 

 Deficiencies of Plants 



It is obvious that a ready means of diagnosing deficiencies of 

 the various mineral constituents of plants is likely to have great 

 economic value, particularly where crop plants are concerned. 

 Where the deficiency of a particular element is great the plant 

 generally displays symptoms which are readily recognizable by 

 an observer with experience of the effects on the species in 

 question of deficiency in that element. These symptoms are 

 often so definite that the resulting condition has a descrip- 

 tive name, and a number of well-defined deficiency diseases 

 of crop plants are known to agriculturists and horticulturists : 

 the most important of these are described in a later chapter. 1 



1 See particularly Wallace (1943, 1944) for diagnoses and coloured 

 illustrations of crop plants of Britain affected by deficiency diseases, and 

 Hunger Signs in Crops by a number of authors (Washington, 1941) for 

 an account, with coloured illustrations, of deficiency diseases of crop 

 plants in the United States. 



