2O6 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Distilled or tap water is sterilized by boiling. Boiling for five 

 minutes will kill ordinary germs if no spores are present. Media 

 are often sterilized by heating in steam at 100 C. Steaming one 

 and one-half hours will sterilize any medium, but this injures 

 some media, especially gelatin. The method adopted in such 

 cases is to steam for 15 minutes on three successive days. This 

 rests on the principle that all bacteria in the non-spored condition 

 are killed the first day, while the spores which are not killed, 

 develop into bacteria and are killed the second or third day. A 

 rapid and effective method of sterilization consists of steaming 

 under pressure at 115 C. from 7 to 15 minutes. 



A variety of media are used for growing bacteria, such as meat 

 extract, broth, blood serum, milk, slices of potatoes, etc. 



Number of Bacteria. One method of estimating the number 

 of bacteria in the soil is as follows : About a gram of the soil is 

 shaken from a weighed tube into a liter of sterilized water, and 

 the tube reweighed. The soil and water are then mixed thor- 

 oughly. Tubes of modified agar 1 are then melted, and one inocu- 

 lated with o.i cc. of water, another with i.o cc. The agar is then 

 poured into flat dishes provided with a cover (petri dish) which 

 have previously been sterilized, and, after the gelatin has 

 hardened, it is set aside for the colonies to develop. This pro- 

 cedure is termed plating. Each kind of bacterium that will grow 

 upon the material used, produces a characteristic group or colony. 



The colonies are then counted. For example, if i cc. of water 

 is used and 100 bacteria developed, the soil contains 100,000 to the 

 gram, since the quantity of water used was shaken with o.ooi 

 gram soil. 



The number of bacteria counted in this way in the soil is some- 

 what variable ; from 8,000 to 6,000,000 per cubic centimeter have 

 been found in the surface soil. The number decreases with the 

 depth, until at the 5th to 6th foot comparatively few are found. 

 The following is an example of such a test : 



1 P. E. Brown, Iowa Research Bulletin No. 2. 



