6 Historical 



Our knowledge of the microbiological population of the soil and 

 its importance in the continuation of life on this planet has been 

 gradually accumulating during the last two and a half centuries, 

 but it is only during the last seventy-five years that rapid progress 

 has been made in dealing with this important branch of biology. 

 Various groups of investigators have contributed to this progress. 



Fig. 3. J. B. J. D. Boussingault initiated during the first half of the nineteenth 

 century some of the most comprehensive investigations in agricultural chemistry 



and physiology. 



The medical bacteriologists were interested in the soil as a medium 

 for the growth and survival of disease-producing organisms. The 

 agricultural chemists were interested in the soil processes that are 

 important for the growth of cultivated plants and that result from 

 the activities of microorganisms. The general bacteriologists, bot- 

 anists, and zoologists were interested in certain special groups of 

 organisms found in the soil, because they presented special problems 

 in microbiology, either from theoretical consideration or from the 

 point of view of practical utilization, as illustrated in the recent 

 search for antibiotic-producing organisms. Finally, the soil micro- 

 biologists proper studied the soil population, either independently 



