44 Microbiological Population of the Soil 



lation, but also for establishing certain important qualitative changes 

 in the population, such as those resulting from fertilization of soil 

 or use of antiseptics. It was also suggested that this method is 

 effective for determining the influence of important plant nutrients, 

 notably nitrogen, phosphorus, and potassium salts, upon the numbers 

 of microorganisms in soil. The method also lends itself readily to 

 the study of the influence of soil treatment upon the rhizosphere or 

 the relationship between plant roots and microorganisms. 



In spite of these favorable results, various investigators reported 

 that the microscopic examination of soil, either by direct staining or 

 by the contact slide method, is inadequate for determining the 

 functions of microorganisms in the soil. These methods were be- 

 lieved, therefore, to be of importance only as supplements to the 

 plate and other culture methods. 



Among the limitations of the contact slide method is the fact that 

 bacteria produce zooglea upon the glass. To overcome this, Cho- 

 lodny proposed the "soil chamber" method, which consists in placing 

 fine particles of soil upon moist slides, keeping the slides in moist 

 chambers, and examining microscopically the organisms growing 

 out of the soil. 



Direct Examination of Soil by Microscope. Kubiena developed 

 a special microscope for the direct examination of soil in an undis- 

 turbed condition. The microbiological population may thus be 

 observed in a natural state. Special surface illumination is attached 

 to the microscope. By means of micromanipulators, some of the 

 larger organisms, notably the fungi, can be removed from the soil 

 for staining purposes and for closer study. This method has not 

 been used very extensively, and only limited information, dealing 

 primarily with the fungi, is so far available. 



The nature of the soil population, as determined by the direct 

 microscopic method, has been summarized by Jones and Mollison. 

 The soil bacteria are largely coccoid and adherent to the humic mat- 

 ter, few or none being attached to mineral particles. They may be 

 in the form of large zoogleal colonies or may consist of smaller 

 clumps or single individuals. Frequently groups of large cocci 

 resembling Azotobacter cells are seen. Long rods are but rarely 

 observed in fresh soil. Fungal mycelium shows variable staining. 

 There is, in fact, strong evidence of correlation of intensity of stain- 

 ing with viability. Progressive loss of the protoplasm from the 

 hyphac, due either to decomposition or to its migration to the hyphal 

 tip, can be frequently observed, most of the hyphal fragments lack- 



