SELMAN A. WAKSMAN 313 



The protozoa occur in the soil in numbers ranging from a few hundred to over a 

 million per gram. The flagellates and amebae are most abundant; the ciliates are less 

 abundant. Nematodes occur in hundreds of millions per acre of soil. The numbers of 

 earthworms and insects also run up to millions per acre. 



The maximum number of organisms is found either at or just below the surface of 

 the soil; then the number diminishes rapidly, so that at a depth of 18-30 inches 

 humid soils contain only very few bacteria and fungi. In arid soils, however, the mi- 

 croflora and microfauna are distributed throughout much greater depths, frequently 

 to 10 feet or more, due to better aeration and to a more uniform distribution of the 

 soil organic matter. The numbers of micro-organisms in the soil vary with the differ- 

 ent seasons of the year, and frequently they may fluctuate even daily, under field con- 

 ditions. There is a rapid increase in the number of organisms in the spring, followed 

 by a drop in the summer, then by another increase in the fall and a second drop in the 

 winter. The summer drop is a result of a lack of sufiicient moisture and the winter 

 drop because of low temperatures. 



ISOLATION AND CULTIVATION OF SOIL MICRO-ORGANISMS 

 METHODS OF ISOLATION 



For the isolation of the great majority of soil micro-organisms, the same general 

 methods are employed as for the isolation of bacteria, yeasts, and fungi from any other 

 substrate such as milk, water, sewage, foodstuffs, etc. However, as a result of the fact 

 that the soil harbors a number of specific organisms, which are not found readily in 

 most of the other substrates and which do not develop at all on most of the common 

 media, special methods must frequently be employed. Various enrichment or elective 

 methods have been utiHzed by Beijerinck and others for the isolation of organisms 

 from soil. These methods consist in preparing a specific medium which allows the 

 development of one particular organism in preference to others. Such a medium is 

 inoculated with soil, and, after successful growth has been obtained, transfers are made 

 to fresh lots of the same medium, with the result that all contaminants, or those 

 organisms which are not able to grow on the particular medium or find conditions un- 

 favorable for their development, are gradually eliminated. Final isolation and purifi- 

 cation of the specific organism is accomplished by the use of special plate methods or 

 by employing high dilutions. A number of bacteria have been isolated from the soil by 

 the use of this method. It is sufficient to mention the symbiotic and non-symbiotic 

 nitrogen-fixing bacteria, the sulphur-oxidizing, the urea-decomposing, the sulphate- 

 reducing, the cellulose-decomposing, the hydrogen bacteria, the methane bacteria, 

 and a number of others. 



Instead of using liquid media to bring about the development of the specific or- 

 ganism, Winogradsky suggested the employment of silica gel media, freed from all 

 traces of impurities, and containing the specific nutrient, which can be acted upon only 

 by the specific organism which is to be isolated. For example, silica gel plates con- 

 taining the necessary minerals, a source of nitrogen, and some ground cellulose on its 

 surface will allow the growth of practically pure cultures of cellulose-decomposing 

 bacteria, when inoculated with a particle of soil containing these organisms. When a 

 silica plate, containing mannitol or glucose as the only source of carbon and no traces 



