MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 373 



1902. He suggested the use of special media for the quantitative 

 estimation of different physiological reactions; thus, making a i per 

 cent solution of peptone and inoculating with equivalent quantities of 

 soil, he caused the decomposition of the peptone and the formation of 

 ammonia, and secured comparisons of the ammonifying power of 

 different soils. In a similar manner he used special solutions for com- 

 paring quantitatively the transformation accomplished by nitrifying, 

 denitrifying or nitrogen-fixing bacteria. 



Remy's method has been extensively tested by Lohnis, Ehrenberg, 

 Lipman and others. It has been shown to possess a serious defect in 

 that it deals with conditions unlike those occurring in the soil itself. 

 For this reason more recent investigations have been carried on in 

 weighed portions of soil rather than in culture solutions inoculated with 

 10 per cent of soil as is done in Remy's method. 



RATE OF OXIDATION OF CARBON. The rate of decomposition of 

 humus or of other organic matter in the soil may be measured, as was 

 done by Wollny, by determining the amount of carbon dioxide evolved 

 in weighed quantities of material kept under definite conditions. The 

 influence of temperature, moisture, aeration, organic matter, anti- 

 septics, etc., has been determined in this manner. The same method 

 may be used in studying decay, and factors influencing decay, in soils 

 in the field. 



More recently Russell and his associates have modified the method 

 in that they have determined the rate of oxidation of carbon not by 

 measuring the carbon dioxide evolved, but by estimating the amount of 

 oxygen absorbed. In either case decay is measured from the carbon 

 standpoint. The method based on this principle should find wide 

 application in future soil fertility investigations. 



Potter and Synder measured the amount of carbon dioxide evolved 

 from sterilized soil when inoculated with soil emulsion or with cultures 

 of molds. The latter produced in nearly all cases as much carbon diox- 

 ide as the soil suspension and in some cases more. This fact led them 

 to suggest that molds are probably active in normal soils. Gainey 

 pointed out that there is a similarity and agreement between the curves 

 representing carbon dioxide and ammonia formation in soils. The 

 relative content and availability of the carbon and nitrogen sources in 

 the soil influence greatly the relative amounts of carbon dioxide and 

 ammonia produced. 



