MICROBIOLOGICAL ANALYSIS OF SOIL 725 



by the soil fungi and actinomyces, but also by the great majority of hetero- 

 trophic soil bacteria, including the nitrogen-fixing organisms, its rate of de- 

 composition is very rapid. It is especially rapid when conditions are favorable 

 for the latter group of organisms, which make them independent of a supply of 

 available nitrogen. 



2. Cellulose. The decomposition of cellulose in the soil is based upon the 

 establishment of a nitrogen minimum. One gram of cellulose, in the form of 

 ground filter paper, is added to 100 grams of soil and the C0 2 evolved is deter- 

 mined; this indicates not only the ability of soil to form C0 2 from cellulose, but 

 also the amount of available nitrogen and phosphate present in the soil. This is 

 due to the fact that the cellulose is decomposed in the soil (with the exception 

 of alkaline or partially sterilized soils) to a large extent by fungi. These rapidly 

 growing organisms consume a great deal of nitrogen in the synthesis of their 

 mycelium, and the nitrogen soon becomes a limiting factor; therefore, the greater 

 the amount of available nitrogen in the soil the larger will be the quantity of 

 cellulose decomposed. The distinctive differences in the curves of C0 2 evolu- 

 tion from glucose and cellulose have been pointed out elsewhere (p. 675). In 

 addition to glucose and cellulose, other substances, like straw, alfalfa meal 

 and dried blood, may be used in the study of decomposition of organic matter in 

 the soil. Cellulose may also be used together with sufficient nitrogen in the 

 form of inorganic salts. 



CELLULOSE DECOMPOSING CAPACITY OF THE SOIL 



Christensen 30 was the first to suggest that the cellulose decomposing 

 power of a soil may serve as an index of soil fertility. A definite amount 

 of the soil to be investigated was placed in Erlenmeyer flasks so as to 

 cover four-fifths of the bottom of the flasks. Water was added from 

 a pipette to the uncovered part of the bottom of the flask. A few strips 

 of filter paper were then pressed upon the soil, the latter being kept 

 moist during the period of incubation. Between 9 to 93 days were 

 required for the complete decomposition of the paper. The physical 

 condition of the soil and its reaction did not influence greatly the 

 cellulose decomposing capacity of a particular soil. 31 The presence of 

 available minerals, primarily phosphates, as well as available nitrogen 

 were found to be of first importance, and, in some cases, the microbial 

 flora; i.e., the phenomenon of cellulose decomposition is influenced by 

 the chemical and microbiological soil conditions. The amount of 

 cellulose decomposed was governed by the available nitrogen and 

 phosphates in the soil. Only in the case of certain peat soils, did the 



30 Christensen, H. R. Ein Verfahren zur Bestimmung der zellulosezersetz- 

 enden Fahigkeit des Erdbodens. Centrbl. Bakt. II, 27: 449-451. 1910. 



31 Christensen, H. R. Studien iiber den Einflusz der Bodenbeschaffenheit 

 auf das Bakterienleben und den Stoffumsatz im Erdboden. Centrbl. Bakt. II, 

 43: 1-166. 1915. 



