52 METHODS OF INVESTIGATING 



But by the time unmistakable symptoms of deficiency have 

 shown themselves it may be late, and, perhaps, too late, to 

 effect a cure of the condition by the application of the deficient 

 mineral; this is likely to be so with annuals such as cereals 

 and leguminous crop plants with a short life period rather than 

 with perennials such as fruit and other trees where the longer life 

 of the plant may provide adequate time for recovery. But 

 even for the latter early diagnosis is obviously desirable in 

 order to avoid a period of feeble growth or poor fruit yield. Also 

 it may be possible that the deficiency of a particular element is 

 insufficient for actual symptoms of a' deficiency disease to 

 develop and yet sufficient to bring about a reduction in the 

 rate of growth and finally in crop yield. 



A second way of determining micro-nutrient deficiency is 

 provided by analysis of plant material by the methods described 

 earlier in this chapter. This might provide very definite evidence 

 of the adequacy or otherwise of the quantity of the various 

 nutrients in the plant, although it would be necessary first to 

 establish the minimum quantities of the respective mineral 

 elements which must be expected in the different organs of the 

 plants of each species at different stages of development. While 

 a certain amount of such information is available it must be 

 admitted that it is far from complete for any one species. The 

 acquisition of the requisite data takes time, but there can be 

 no doubt that the necessary information will ultimately be 

 obtained. 



A third method of diagnosing mineral deficiencies, and one 

 which should enable this to be made early, consists in intro- 

 ducing a solution of the salt of the element in question, or even 

 the solid salt, into the plant and observing the reaction. The 

 introduction of the salt into the plant is generally spoken of as 

 'injection'. The generally accepted meaning of this word is the 

 forcing of material into the organism under pressure, whereas 

 in practice the plant is generally allowed to absorb the solution 

 through a cut surface, or even through an intact leaf, without 

 the application of pressure. However, there is no other simple 

 term to denote this process, and we may follow Roach, who has 

 developed this method for diagnosis of trace-element deficiencies, 

 in extending the use of the term 'injection' to include 'the 



