3H 



INORGANIC NUTRITION AND METABOLISM 



Figure 4. The form of assay 

 curves, employing Aspergillus 

 niger, for zinc, molybdenum, 

 and copper. Redrawn from 

 D. J. D. Nicholas, The 

 Analyst, Vol. 77, p. 629-642 

 (1952), by permission of the 

 Society for Analytical Chem- 

 istry. 



Concentration of elemeat 



the minerals actually available to a crop plant. The assumption made, 

 therefore, is that the fraction of a given mineral which the plant cannot 

 absorb is equally unavailable to the fungus. Consequently, the major 

 criterion for the practicability of a bioassay must be the degree to 

 which the assay results predict the behavior of crop plants in a given 

 soil; correlation with chemical determinations is of secondary impor- 

 tance. Correlative studies have usually shown at least fair agreement 

 between growth of Aspergillus niger and growth of higher plants in 

 soils deficient in phosphorus, potassium, zinc, copper, and molybdenum 

 (47, 73, 147, 150, 168). The fungus is less suitable for manganese deter- 

 mination because metabolic acids make more manganese available to it 

 than is available for higher plants (153). 



Typical assay curves are shown, in simplified form, in Figure 4; it is 

 to be noted that the response is not rectilinear. Detailed methods are 

 described by Donald et al. (47), Gerretsen (73), Mulder (151), and 

 Nicholas (168). 



14. PROBLEMS IN MINERAL NUTRITION 



Two general approaches may be used in determining the essentiality 

 of a given element. One may design a medium free of the element in 

 question and containing all other materials necessary for growth. Es- 

 sentiality is shown by a quantitatively determined growth response to 

 graded amounts of some soluble compound containing the element. 

 Obviously, the medium must be purified rigorously for the study of 



