MICROBIOLOGICAL ANALYSIS OF SOIL 729 



1 . One gram of finely cut or well ground filter paper is well mixed with 100 grams 

 of fresh sieved soil. This is placed in a tumbler, brought to optimum moisture, 

 covered, and incubated for 42 days, at 25° to 28°C. with frequent additions of 

 water to keep at optimum moisture. The amount of residual cellulose is deter- 

 mined by the method of Charpentier in the soil which is first air dried. The 

 residual cellulose is then subtracted from the amount of cellulose originally present 

 in the soil, which is determined by extracting 20 grams of the original soil to 

 which 200 mgm. of the paper has been added. The amount of cellulose actually 

 decomposed in the soil is thus obtained. 



2. One gram of well ground filter paper and 100 mgm. of sodium nitrate are 

 added to 100 grams of soil. The mass is then well mixed and placed in a tumbler, 

 brought to optimum moisture, covered, and incubated for 15 days. The amount 

 of cellulose decomposed is determined as in the case of the first method. 



3. One hundred grams of soil, 200 mgm. of CaC0 3 , 50 mgm. K 2 HP0 4 , 25 mgm. 

 MgS0 4 , with and without one gram of ground dry filter paper are mixed in 

 tumblers. These are placed in a respiratory apparatus and the amount of COi 

 given off in fourteen days is determined. The excess of C0 2 produced in the 

 soil containing the cellulose over that produced in the soil containing the 

 minerals only, and the amount of cellulose decomposed will serve as an index of 

 the cellulose decomposing power and, ipse facto, of the available nitrogen in the 

 soil. For every milligram of nitrogen that is available in the soil or that can 

 become available in the given period of time, about 50 mgm. of cellulose are 

 decomposed. 



NITROGEN-FIXING AND MANNITE DECOMPOSING CAPACITY OF THE SOIL 



The principle of the various methods used for the study of nitrogen 

 fixation can be summarized as follows. A readily available source of 

 energy, chiefly mannite or glucose, is added to the soil or to a solution 

 inoculated with soil ; the amount of available nitrogen in the soil is very 

 limited, so that the fungi and heterotrophic non-nitrogen fixing bacteria, 

 which would otherwise be capable of consuming the mannite or glucose, 

 cannot do that extensively. The amount of glucose or mannite 

 commonly used in the laboratory studies (1 to 2 per cent) is in great 

 excess, so that the amount of available nitrogen is far from sufficient 

 for supplying the requirements of the non-nitrogen fixing organisms. 

 The bacteria, which are capable of obtaining nitrogen from the gaseous 

 form, can readily utilize mannite or glucose as sources of energy. It 

 has been shown elsewhere (p. 444) that the addition of celluloses and 

 cellulose-rich substances to the soil greatly stimulates the development 

 of fungi, especially in the presence of available nitrogen. The addi- 

 tion of glucose to the soil does not affect greatly the development of 

 these organisms, but brings about an abundant multiplication of 

 bacteria, especially the nitrogen-fixing forms. 



In the presence of an available source of energy, the nitrogen-fixing 



