PRINCIPLES OF MICROBIAL METABOLISM 



377 



stimulative and injurious substances may be formed constantly. 18 It 

 has even been claimed that soil microorganisms produce substances 

 (nucleic acids) which stimulate the growth of cultivated plants. 19 



Chemical composition of the microbial cell. The presence of a certain 

 number of organisms in a given soil and at a given time is not due to a 

 mere accident, but because a definite amount of energy, nitrogen and 

 minerals are made available for their growth and reproduction in a given 

 period of time, and as a result of definite environmental conditions. 

 A change in the energy supply and in the environment will change not 



e i -z 3 ', 



7 e ? 10 11 11 11 J* /} IS 'I IB '» 10 II 11 Z3 2if 15 21 ZT ZS if 10 Jl 1Z SI J* IS 



Days. 



Fig. 15. Rate of oxidation of ammonia to nitrate, as shown by disappearance 

 of ammonia in soil (after Miyake and Soma). 



only the numbers of microorganisms, but also the nature of the soil 

 population. Different numbers and different kinds of microorganisms 

 will be found in a given soil, at different periods, because varying 

 quantities of nutrients are made available. When a soil is air-dried 

 and again moistened, there is an increase in the numbers and activi- 

 ties of microorganisms, because the process of drying results in ren- 

 dering a greater amount of the inorganic and organic matter available. 



18 Pringsheim, E. G. ITber die gegenseitige Schadigung und Forderung von 

 Bakterien. Centrbl. Bakt. II, 51: 72-85. 1920. 



19 Mockeridge, F. A. The formation of plant growth-promoting substances 

 by microorganisms. Ann. Bot., 38: 723-734. 1924. 



