166 LABORATORY GUIDE IN BACTERIOLOGY 



to obtain an average representation of the soil. The 

 cylinders are wrapped in sterile paper provided for 

 that purpose and taken to the laboratory. Take 

 samples in this manner from various kinds of soil, viz., 

 garden soil, sand, loam, clay, manured fields, etc. 



EXERCISE 2. PREPARATION OF SAMPLES FOR BAC- 

 TERIOLOGICAL EXAMINATION 



1. The samples from the same kind of soil are mixed 

 in a sterile mortar. 



2. Place part of each of these composite samples 

 in a sterile glass tube. 



3. Weigh accurately 10 grams from each composite 

 sample and dry in a hot-air oven for one hour at 100 C. 

 Cool and place in a desiccator. After 24 hours weigh 

 again and calculate the percentage of dry matter 

 present. 



EXERCISE 3. PREPARING SOIL SUSPENSIONS AND 

 DILUTIONS AND POURING PLATES 



1. Prepare .sterile dilution flasks. Fill 100 c.c. 

 water in Erlenmeyer flasks of about 150 c.c. capacity. 

 Sterilize these in the autoclave for 10 minutes. As 

 there is a loss by evaporation during the process of 

 sterilization the remaining amount is approximately 

 99 c.c. 



2. Weigh the tube containing the sample of soil. 



3. Remove about 10 grams by shaking the tube or 

 by means of a sterile knife or spatula. Place in a 

 sterile mortar. 



4. Weigh the tube again and determine the precise 

 amount of soil removed. 



5. Grind the soil sample in the mortar with some 



