CHAPTER I 



Numbers of Different Groups of Microorganisms Found in the 

 Soil and Methods of Determination 



The occurrence of microorganisms in the soil. The microorganisms 

 present in the soil belong, in an uneven proportion, to the plant and 

 animal kingdoms, the former including the large majority both in 

 numbers and in kinds. Chart 1 gives a visual representation of the 

 relationships of the various groups of soil microorganisms. The 

 relative importance in the soil, however, both as to numbers and 

 physiological activities, varies with the different groups. 



The animal world is represented in the soil by the protozoa, nema- 

 todes, rotifers, earthworms and various other worms as well as insects. 

 The nematodes occur abundantly in all soils, but especially in green- 

 house soils and certain infested field soils. Large numbers as well as 

 numerous species of amoebae, ciliates and flagellates represent the 

 protozoa in the soil. 



The microscopic plant world is represented in the soil by the algae, 

 fungi and bacteria, named in the order of their increasing importance 

 of numbers and activities. Among the algae, the Cyanophyceae and 

 Chlorophyceae are best represented in the soil. The soil fungi can be 

 subdivided further into three groups : 



1. Yeasts and yeast-like fungi, like the Monilia and Oidia (these 

 two groups may, however, be classed with the true fungi). 



2. Molds and other true fungi. Here we find the Mucorineae repre- 

 sented by the extensive genera Rhizopus, Mucor, Zygorhynchus and 

 other Phycomycetes; various Ascomycetes, including the genus Chae- 

 tomium and other genera; Hyphomycetes represented by the Mucedi- 

 naceae (Aspergillus, Penicillium, Sporotrichum, Botrytis, Trichoderma, 

 Verticillium, etc.), Dematiaceae, Stilbaceae and Tuberculareaceae. 

 The Basidiomycetes are probably represented abundantly in the soil 

 by the sterile mycelium as well as by some of the mycorrhiza fungi. 



3. Actinomyces. Ten to 50 per cent of the colonies developing from a 

 soil on the common agar or gelatin plate belong to this important group 

 of soil organisms. They are generally classified by bacteriologists with 



3 



