BACTEEIA OF THE SOIL 265 



present in the soil and undoubtedly is one of the important 

 factors in bringing about the solution of materials necessary 

 for the growth of higher plants. 



The Decomposition of Insoluble Carbohydrates in the 

 Carbonaceous Matter in the Soil. — ^A considerable pro- 

 portion of the carbonaceous material added to the soil is in 

 the form of cellulose and closely related compounds. This 

 is not readily attacked by most kinds of bacteria, but there 

 exist in the soil certain specialized forms which decompose 

 cellulose readily. Members of the genus Actinomyces are 

 particularly active in this respect, causing rapid digestion 

 of cellulose. Certain molds are also active, particularly in 

 acid soils. Then, too, a considerable number of species of 

 bacteria have also been isolated from the soil, capable of 

 bringing about cellulose digestion. They belong to a 

 variety of genera. They may be recognized in the labora- 

 tory by their ability to grow upon an agar containing finely 

 divided cellulose and to produce a clearing of the medium. 

 By the destruction of cellulose in the soil, decomposition 

 products such as the simple soluble carbohydrates and 

 organic acids, are produced which may be utilized by many 

 other soil forms. 



The changing organic matter present in soil is usually 

 termed collectively "the soil humus." Most plant residues 

 added to soils are comparatively high in carbon and poorer 

 in nitrogen. In the process of decomposition carbon, in the 

 form of carbon dioxide, is given off more rapidly than is 

 nitrogen. In consequence, there is a tendency for the 

 humus, as it becomes older, to show a smaller ratio of nitro- 

 gen to carbon. This carbonaceous material present in soil 

 is of significance from many points of view. It ameliorates 

 the physical condition of the soil and furnishes directly and 

 indirectly from the products of its decomposition food 

 material for many of the nitrogen-fixing and other soil 

 bacteria. It is made up in part, of course, of the bodies of 



