THE STUDY CF MICROORGANISMS IN THE SOIL 93 



nature and abundance of the various types of bacteria thus 

 depends both upon the available nutrients present in the soil and 

 upon the soil environmental conditions, such as reaction, aeration, 

 moisture content, abundance of organic matter, etc. The plate 

 method of counting gives only a fraction of the total number of 

 bacteria present in a given soil. This is due to the fact that 

 the many types of bacteria such as the autotrophic, anaerobic, 

 nitrogen-fixing ones, etc., do not develop on the common media. 



2. Fungi. — Although the bacteria from the largest group of 

 organisms in most soils, the fungi may be present in considerably 

 greater bulk in some soils, such as acid forest soils or heavily 

 manured, acid garden and field soils. This is due to the consid- 

 erable difference in size of a piece of fungus mycelium and a 

 bacterial cell. The abundance of fungi in the soil can be deter- 

 mined both by the plate and the microscopic methods. A colony 

 may develop either from a spore or from a piece of mycelium. 

 Spore fungi, like most Hyphomycetes, form a mycelium which 

 readily breaks up into fragments. Others, like the Phycomycetes 

 form a unicellular mycelium. In the case of these organisms, the 

 number of colonies developing on a plate may be far from repre- 

 senting their actual abundance. The fungi are represented in the 

 soil by (a) the filamentous fungi or molds, (b) the actinomyces, or 

 ray fungi, and (c) the yeasts. In acid forest soils, there is an 

 extensive development of fungus mycelium, belonging to Basidio- 

 mycetes and other fungi, which do not readily form any fruiting 

 bodies. Many of these produce '^mycorrhiza" with the roots of 

 various trees, and a number of other perennial and annual plants. 

 The relative abundance of these fungi can be obtained only by the 

 aid of the microscope. 



3. Algae. — The chlorophyll-bearing microscopic plants do not 

 need any carbon compounds as sources of energy. They obtain 

 from the soil only nitrogen and minerals while the carbon is 

 derived from the CO2 of the atmosphere, using photosynthetic 

 energy. The algae are represented in the soil by the (a) Cyano- 

 phycese, or blue-greens, (h) Chlorophycese, or grass-greens, (c) and 

 Diatomaceae, or diatoms. Many of the algae are able to live at 

 lower depths of soil and are capable of obtaining their carbon 

 and energy from complex organic compounds. The number 

 of algae in the soil is best determined by the dilution method. 



