122 BACTERIOLOGY OF THE SOIL 



From this weigh out 15 or 20 gr. and add it to 300 or 400 c.c. 

 of sterilized tap water in a J-litre flask. Shake well, and if 

 necessary break up the soil with a sterilized glass rod. 



Take up with a wide bored sterilized pipette 25 c.c. of the 

 turbid water, and add it to another flask containing 300 or 400 

 c.c. of sterilized tap water. 



From this second flask take 25 c.c. and transfer to a third, and 

 from the third to a fourth flask. 



A. Inoculate .a Petri dish of the Heyden agar medium with 

 i c.c. of the water from the fourth flask and incubate at 20 C. 

 Count the colonies which appear in ten days, and calculate the 

 number of bacteria present in i gr. of the original soil. 



B. Inoculate a Petri dish of 



(i) Nutrient gelatine. 



(ii) Nutrient agar neutralized or rendered very slightly 

 alkaline with a drop or two of scjdium carbonate 

 solution, with i c.c, of the " soil-dilute," as in A. 

 Incubate at 20 for ten days. 



Examine every day and count the colonies. Use a Pake's 

 disk (Fig. 45) beneath plate to facilitate counting. 



The colonies of liquefiers on the gelatine medium may be 

 killed by touching them with a " caustic pencil " or stick of silver 

 nitrate. 



Compare the results obtained with the three media. 



Ex. 69. Repeat the above with 



(i) Soil from a well- manured garden, 

 (ii) an arable field, 

 (iii) an old pasture. 



3. Kinds of Bacteria in the Soil. A large number of 

 kinds of bacteria have been isolated from the soil. Some 

 of them are added to it in the manures which are applied 

 to the land, others find their way into it in drainage water, 

 sewage, and rain. Many of the species are casual introduc- 



