Wallace: The Method of Visual Diagnosis 



h^ Excesses— causing direct toxicities and induced deficiencies. 



c) Relationships of trace elements to other nutrients. 



d) Soil acidity efiEects. 



Deficiencies— Many effects of deficiencies of individual trace 

 elements, like those arising from deficiencies of the various major 

 elements, are characteristic and can be used for diagnostic pur- 

 poses. Well known examples are 'Heart Rot' of sugar beet and 

 mangolds, 'Cracked Stem' of celery and 'Corky Core' of apples due 

 to boron deficiency; 'Litde Leaf of fruit trees due to zinc de- 

 ficiency; and 'Wither Tip' of oats due to copper deficiency. In 

 our investigations we have been able to distinguish many charac- 

 teristic effects resulting from deficiencies of iron, manganese and 

 boron, some of which are illustrated here today, whilst our prelimi- 

 nary studies with molybdenum promise to be equally fruitful of 

 results. It should also be mentioned that in some instances more 

 than one deficiency may be recognised visually on a single plant. 



I should like to stress the point that a knowledge of these effects 

 may give considerable guidance in laboratory studies and may also 

 save considerable time, effort and expense in field investigations. 

 These two points will become evident in considering some of the 

 points which follow. 



Excesses.— The effects of excesses of certain trace elements have 

 been examined on a number of plants and here again many effects 

 produced have been found to be characteristic and easily recog- 

 nised visually for various plants. Thus excess of boron produces 

 a very characteristic type of marginal scorch on the older leaves of 

 many plants. Excess of manganese produces spotting of the 

 leaves of cereals and potato; stem and petiole lesions on potato and 

 tomato; chlorotic mottling of the leaves of sugar beet, runner bean 

 and many of the Brassicae; and also incurling and necrotic spot- 

 ting of the leaf margins, and distortion and suppression of the 

 laminae of the leaves of Brassicae (2, 3). Excess of aluminium 

 results in visual effects on leaves which are similar to those due to 

 phosphorus deficiency whilst roots are characteristically stubby and 

 lacking in fibre (10). 



It has also been shown by the visual method that two distinct 

 effects may result from excesses of trace elements, namely direct 

 toxic effects, such as those above, and induced deficiencies of other 

 elements. Thus it has been demonstrated that an excess of man- 

 ganese, zinc, copper, cobalt, nickel or chromium, in addition to 

 producing direct toxic effects, may also induce a deficiency of 



