712 PRINCIPLES OF SOIL MICROBIOLOGY 



are of importance in this connection. Engberding, 7 however, found 

 that whenever a difference was observed as due to cropping, it could 

 be accounted for by the difference in the moisture content of the soil. 

 The addition of organic matter to the soil, either in the form of stable 

 manure, green manure and plant stubble, is known to have a decided 

 stimulating effect upon the number of microorganisms. After most 

 of the available energy materials have been decomposed the numbers 

 begin to fall again, so that in a few months the level of the control may 

 be reached. Soluble inorganic nitrogen salts and minerals also exert 

 a stimulating effect upon the numbers of microorganisms. 



The correlation between the numbers of microorganisms in the soil and 

 soil fertility has met, on the one hand, with a certain amount of criti- 

 cism, but, on the other hand, has yielded some very interesting results 

 in the hands of a number of investigators. L6hnis 7a did not consider 

 the determination of numbers of microorganisms in the soil as bearing 

 upon soil fertility processes. Certain other investigators 8 could not 

 find any correlation between soil fertility and results obtained by 

 physiological methods (ammonification, nitrification). 



On the other hand, a great deal of confidence was placed 9 in the 

 determination of the numbers of microorganisms in the soil by the 

 plate method, with the idea of correlating these results with soil fer- 

 tility. It was believed that an insight into some of the differences in 

 the productivity of different soils could thus be obtained. A definite 

 correlation was found to exist 10 between crop yield, oxidizing power of 

 soil, nitrate production and numbers, but not between crop yield and 

 ammonia accumulation. It was concluded that the determination of 

 numbers of microorganisms in the soil by the plate method, allowing 

 for the variability of the methods used and of the soil, can serve as a 



7 Engberding, 1909 (p. 14). 



7a L6hnis, F. Landw. Jahrb. 42: 751-765. 1926. 



8 Maassen and Behn. Zur Kenntnis der bakteriologischen Bodenuntersuch- 

 ungen. Arb. Biol. Reichsanst. Land u. Forstw., 11: 399-505. 1923. 



9 Fischer, H. Besitzen wir eine brauchbare Methode der bakteriologischen 

 Bodenuntersuchung? Centrbl. Bakt. II, 23: 144-159. 1909. 



10 Neller, J. R. The oxidizing power of soil from limed and unlimed plots 

 and its relation to other factors. Soil Sci., 10: 29-39. 1920; Noyes, H. A., and 

 Conner, S. D. Nitrates, nitrification and bacterial contents of five typical 

 soils as affected by lime, fertilizer, crops and moisture. Jour. Agr. Res., 16: 

 27-42. 1919; Waksman, S. A. Microbiological analysis of soil as an index of 

 soil fertility. III. Influence of fertilization upon numbers of microorganisms 

 in the soil. Soil Sci., 14: 321-346. 1922. 



