20 



SOIL BACTERIOLOGY 



to 40° C, to each plate. A blank plate or control should 

 be poured with each series. In case the medium is turbid, 

 heat slowly, allowing the deposit to settle. Use only the 

 clear portion of the medium for pouring plates. 



8. Immediately after adding the culture-medium rotate 

 each plate to secure a uniform mixture. Allow agar plates 

 to harden on a level surface. 



Fig. I.— Agar plate showing a common form of spreading colonies found 



in soil. 



9. Agar plates should be inverted and incubated under 

 a moist chamber at 28° C. The time of incubation will 

 depend upon the different culture-media. After five to 

 ten days count the number of colonies on each plate. If 

 the colonies are not too thick, it is well to dot each one with 

 a pen. When the colonies are too thick to count easily, 

 use a hand lens and counting plate. 



