CARBON AND NITROGEN CYCLES IN THE SOIL 187 



Subsequent developments have been entirely on the 

 bacteriological side. A number of organisms are now known 

 to produce ammonia from complex nitrogen compounds, but 

 soil bacteriologists, notably J. G. Lipman and his school at 

 New Jersey, have generally preferred to study the group as 

 a whole, rather than isolate and study individual members. 

 The method consists in inoculating soil into various arbitrary 

 culture media, each designed to favour one group only of 

 organisms. Some of the results obtained are discussed in 

 Chapter VII. ; they show the method has value as a bacterio- 

 logical test, but it has thrown little or no light on the processes 

 going on in the soil. Indeed, so dependent is bacterial activity 

 on temperature, concentration, reaction of medium (whether 

 acid or alkaline), and other conditions, that it may be doubted 

 whether any method of study, except in the actual soil itself, 

 will further our knowledge of the reaction very much. 



Of the few attempts to study the individual species of 

 organisms concerned in ammonification H. J. Conn's is per- 

 haps the most notable. Contrary to the accepted view he 

 claims that ammonia formation is mainly brought about by 

 non-spore formers: B. mycoides^ generally regarded as one of 

 the most common ammonia producers in the soil, h dismisses 

 as ineffective. He maintains that of the eight ammonifiers 

 studied by Marchal only one, B. fluorescens liq. (a non-spore 

 former), is a typical soil organism. He describes in detail 

 two organisms, Ps. fluorescens and Ps. caudatus^ which, while 

 not very numerous in unmanured soil, multiply vigorously 

 on addition of farmyard manure and also produce ammonia 

 (70*:). Waksman has adduced evidence (see p. 259) that fungi, 

 and especially actinomycetes, are active ammonia producers 

 in soil. 



Nitrification. 



The ammonia formed by the action of soil bacteria, or 

 added in manures, is changed to carbonate, which is then 

 rapidly converted by Nitrosomonas into nitrite, and this by 



1 Really a group, not a single organism. 



