THE STUDY OF MICROORGANISMS IN THE SOIL 95 



The microorganisms are responsible for the various biochemical 

 processes going on constantly in the soil and which lead to the 

 liberation of nutrients essential for the growth of higher plants. 

 These nutrients include the carbon, as carbon dioxide, the 

 nitrogen as ammonia and nitrate, the minerals, especially 

 phosphorus and potassium. The following exercises are planned 

 to show how the microorganisms liberate those nutrients, as well 

 as the nature of the organisms themselves and methods of 

 study. 



MICROSCOPIC EXAMINATIONS OF MICROORGANISMS 



Exercise 1 

 Examination of Living Microorganisms in Hanging Drops 



Place a small drop of the liquid to be examined in the center 

 of a cover glass without spreading. Smear vaseline around 

 the concavity of a hollow-ground slide and invert this slide over 

 the cover glass with the drop so that the cover glass adheres 

 to the slide. Turn over the sHde and see that the drop does not 

 touch the sHde. Examine the hanging drop with a low- and then 

 with a high-power objective. To secure the best results reduce 

 the illumination. The addition of a small amount of nigrosin 

 (saturated aqueous solution of nigrosin B) to the hanging drop 

 makes the bacteria stand out as clear spots in a dark background. 

 It is advisable to focus on the edge of this drop. 



While this method of examination is primarily for detecting 

 motihty and Brownian movement, it is useful in a study of general 

 morphology. 



Examine: 1. Hay infusion. 



2. Pure culture of yeasts and bacteria. 



Exercise 2 



A. Examination of Bacteria in Congo Red or Nigrosin 

 Preparations 



This method of examination as described below has many 

 advantages; it is rapid and does not alter the size of the organisms. 

 It is especially useful for liquid cultures. The bacteria appear as 

 clear spots in a dark background. If Congo red is used the dead 

 cells appear as faintly colored, the living as colorless. Because 



