92 LABORATORY MANUAL OF MICROBIOLOGY 



Method II 

 The following solutions are required: 



A. Aniline 120 gm. 



Hydrochloric acid (commercial) 180 gm. 



Water 1,000 cc. 



B. Sodium dichromate 120 gm. 



Hydrochloric acid 100 gm. 



Water 1,000 cc. 



Solution A should be applied with a brush to the fresh smooth 

 surface and allowed to dry overnight. The color will turn 

 bright yellow. Solution B should then be spread on the wood, 

 which will turn dark and be very streaky at first. After this 

 second coat dries the surface should be rubbed with vaseline, 

 motor oil, or paraffin. Vaseline seems to be preferable. 



GENERAL CHARACTERISTICS OF THE SOIL POPULATION 



Every particle of soil harbors numerous microorganisms, which 

 vary in kind, nature of nutrition and, therefore in the biochemical 

 processes which they bring about. Although one set of condi- 

 tions may favor the activities of one group of organisms in prefer- 

 ence to others, no soil is known in which only a single species of one 

 organism is found. The actual number of cells may vary from a 

 few hundred thousands per gram in certain very poor sandy soils 

 and in bog soil to many billions in good garden, field, and green- 

 house soils. The number of species varies from a few types of 

 fungi and bacteria in marshy lands or in very acid forest soils to 

 thousands of species in the garden and field soils. 



The soil population can be readily divided into the following 5 

 groups : 



1. Bacteria. — This group is usually the most numerous in the 

 soil, both in the total number of cells and in the number of species. 

 Morphologically, they comprise the cocci, non-spore forming 

 and spore-forming bacteria and spirilla. Some bacteria, forming 

 thin, long, flexuous, undulating cells are often referred to as Spiro- 

 chsetes, but these soil forms belong to the bacteria rather than 

 to the protozoa. These numerous soil bacteria vary consider- 

 ably in their nutrition and in their response to environmen- 

 tal conditions. We have aerobic and anaerobic forms, auto- 

 trophic and heterotrophic bacteria, those that are resistant to 

 acids and those that do not grow at a pH less than 6.0. The 



