MICROBIOLOGICAL ANALYSIS OF SOIL 711 



the transformations take place within a definite length of time, these should not 

 be too rapid, so that the original differences, due to bacterial activities, should not 

 become obliterated. On the other hand, definite transformations should take 

 place at least within one month. 



The soil method was soon substituted 6 for the solution method, for 

 the biochemical soil investigations. Here, the substance is added to a 

 certain amount of soil, well mixed and kept at optimum moisture and 

 temperature, for a certain period, at the end of which the chemical 

 change produced is determined. 



NUMBERS OF MICROORGANISMS IN THE SOIL 



The direct microscopic method has not as yet been sufficiently 

 developed so that it cannot be used readily for determining the total 

 numbers of microorganisms in the soil. So far we have to depend on 

 the plate methods with all its numerous limitations for the information 

 on the relative abundance of microorganisms. The variability of the 

 soil, methods of sampling, preparation of media and plates, and other 

 details for determining the number of microorganisms have been 

 recorded in detail elsewhere (p. 12). Synthetic media should be 

 employed for the soil bacteria and actinomyces, using 8 to 10 plates 

 for each soil sample and taking at least 4 or 5 composite samples 

 from each field or plot. The fungi should be determined on the 

 same samples of soil, using a different acid medium and a dilution of 

 1:100 or thereabouts of that employed for the determination of 

 bacteria. 



Treatment of soil which brings about differences in fertility also 

 results in decided differences not only of the total number of microor- 

 ganisms in the soil, but also of the relation between the different groups 

 of organisms. The system of cropping and the nature of the crop grown 



Bodenfruchtbarkeit. Landw. Jahrb., 33: 1-139. 1904; Lohnis, F. Unter- 

 suchungen iiber den Verlauf der Stickstoffumsetzungen in der Ackererde. Mitt, 

 landw. Inst. Leipzig, H 7: 1-103. 1905. 



6 Stevens, F. L., and Withers, W. A. Studies in soil bacteriology. Centrbl. 

 Bakt. II, 23: 355-373,776-785. 1909; 25: 64-80. 1910; Lemmermann, O., Fischer, 

 H., Kappen, H., and Blanck, E. Bakteriologischchemische Untersuchungen. 

 Landw. Jahrb., 38: 319. 1909; Koch, A., and Petit. t)ber den verschiedenen 

 Verlauf der Denitrifikation im Boden und in Fliissigkeiten. Centrbl. Bakt. II, 

 26: 335-345. 1910; Vogel. Beitn'ige zur Methodik der bakteriologischen Bod- 

 enuntersuchung. Centrbl. Bakt. II, 27: 593-605. 1910; Lipman, J. G., and 

 Brown, P. E. Experiments on ammonia and nitrate formation in soils. Centrbl. 

 Bakt. II, 26: 590-632. 1910. 



