388 MICROBIOLOGY OF SOIL 



Furthermore, apart from the variations from species to species, differ- 

 ences have been observed by Marchal and many other investigators 

 between one strain and another of any single species isolated from the 

 same or different soils. It must be remembered, therefore, that in the 

 study of ammonification in soils and culture solutions, due considera- 

 tion should be given to differences in physiological efficiency as they are 

 manifested by strains and species of microorganisms. 



Apart from the ammonifying bacteria already mentioned there is a 

 group of organisms studied by Muller, Pasteur, van Tieghem, Leube, 

 Miquel, Beyerinck and others. These are the so-called urea bacteria, 

 capable of intensive transformation of urea and allied compounds into 

 ammonium carbonate, by means of the enzyme urease. 



NH 2 



CO + 2 H 2 = (NH 4 ) 2 CO 3 



NH 2 



Morphologicall ythese organisms include spherical and rod forms, 

 spore-bearing and non-spore bearing species. Most of the urea bacteria 

 are particularly prominent in the transformation of animal manures. 



Ammonifying Efficiency. Lipman and Burgess have found marked 

 differences in the ammonifying efficiency of fifteen organisms in pure 

 cultures using peptone, bat guano, sheep and goat manure, dried 

 blood, tankage, cottonseed meal and fish guano. The nature of the 

 soil as well as the nature of the nitrogenous material markedly modify 

 an organism's ammonifying power. B. tumescens on the whole appears to 

 have been the most efficient organism tested. Comparing these findings 

 with those of Marchal the former have obtained results in soils, while 

 the latter 's work was with solution cultures, the application of which 

 to soil conditions is not always permissible. In point of fact the am- 

 monifying efficiency of organisms is greater in sandy soil and possi- 

 bly in others than in solutions, as Lipman and Burgess have obtained 

 a transformation of 41.98 per cent of peptone in nitrogen and 36.06 

 per cent of bat guano nitrogen into ammonia by Sarcina lutea and 

 B. mycoides, respectively, in twelve days at temperatures between 

 27 and 30, while Marchal obtained similar transformations in 

 thirty days at 30 in albumen solutions. 



