Rothamsted Symposium on Trace El ements 94 



soil. When the experimental results are shown in a system of 

 co-ordinates where the ordinate expresses the amount of dry mat- 

 ter while the abscissa gives the amount of plant nutrient absorbed, 

 the experimental results present an S-shaped curve, the point of 

 inflexion of which corresponds with the minimum of the other 

 curve showing the relation between relative content and the 

 amount given to the soil of the microelement in question or 

 the dry matter produced. 



We have observed such curves when the soils in which the 

 plants grow pass from very deficient to less deficient and then to 

 a rich condition of the microelement in question. This has been 

 found both at maturity and at other stages of growth, and for 

 barley both in the leaves and in the grain. These facts can be 

 explained especially by the variations in the content of carbohy- 

 drates at different levels of manuring. 



These observations may explain why, when sampling crops for 

 chemical analyses of microelements, it is often found that the 

 sample which represents a crop seemingly most deficient in, for 

 instance copper, has the highest or about the same percentage of 

 this nutrient in the dry matter as a crop less deficient or not defi- 

 cient. 



There is, moreover, the possibility that the relative content of 

 other plant nutrients may vary simultaneously in the same way, 

 and this further complicates the problem of the use of chemical 

 plant analyses alone as indicators of a deficiency of a given micro- 

 element. This problem is at present being investigated. It 

 seems necessary to use chemical plant analyses with caution and 

 some reservation as a diagnostic criterion of the deficiency of soils 

 and crops in plant nutrients (both macro- and microelements). 

 They should not be used alone, but always in connection with 

 other methods of observation. 



