ESSENTIAL NONMETALLIC ELEMENTS 



103 



cated than this simple theory Yv'ould predict. It should be emphasized 

 that nitrates and ammonium salts have opposite effects on the acidity of 

 culture media. Other conditions being equal, as nitrate ions are con- 

 sumed, the culture medium becomes more alkaline, while as ammonium 

 ions are utilized, the culture medium becomes more acid. 



Before considering the ammonium salts of the organic acids, the use of 

 ammonium nitrate should be mentioned. Both ions contain nitrogen, a 

 feature which has led many investigators to use it in media. If a fungus 

 is able to utilize both kinds of nitrogen, the pH of the medium will be 

 somewhat stabilized. This salt should not be used if the purpose of an 

 experiment is to determine whether a fungus can utilize either one or the 

 other or both forms of nitrogen. Some fungi apparently use nitrate 

 nitrogen in preference to ammonium nitrogen when both are supplied in 

 the medium. Fusarium lini appears to be such a fungus (Wirth and 

 Nord, 1942). 



Table 19. The Effect of Various Organic Acids on the Growth of Four 

 Fungi on Media Containing Ammonium Nitrate 

 Initial pH 5.5. Figures are milligrams of mycelium produced. (Leonian and LiUy, 

 Am. Jour. Botany 27, 1940.) 



M etarrhizium glutinosum {Myrothecium verrucaria) grew well on nitrate 

 nitrogen alone and poorly on ammonium nitrogen (Brian et al., 1947). 

 Ammonium nitrogen inhibited growth of this fungus, whether nitrate was 

 present or not. Growth was equally poor on ammonium nitrate and 

 ammonium sulfate. Since this fungus grew well on media containing 

 nitrate as the sole source of nitrogen, these authors have questioned the 

 common belief that all fungi which are able to utilize nitrate nitrogen can 

 also utihze ammonium nitrogen. Most fungi appear to utihze ammonium 

 nitrogen before nitrate nitrogen when both are supplied in the medium, 

 but this is not universal. Rippel (1931) found the pH of the medium to 

 determine which form of nitrogen was utilized by Aspergillus niger and 

 A. oryzae. Additional examples are given by Foster (1949). 



The utilization of ammonium and some forms of organic nitrogen may 



