BACTERIA IN THE SOIL 83 



are the chief cause of the liberation of free nitrogen into the atmo- 

 sphere. Finally, we unfortunately find pathogenic or disease bacteria, 

 the chief being the bacilli which cause the diseases known as lock-jaw, 

 quarter-evil or black leg, and malignant oedema. These are natural 

 to the soil and form spores, so that they are able to retain their vitality 

 unimpaired for a long time when the conditions are not favourable for 

 their growth. At the same time it must not be imagined that the 

 bacteria which cause other diseases are absent from the soil. Thus, a 

 number of typhoid bacilli find their way to the soil owing to the 

 deposition of sewage and animal excreta on its surface. It has been 

 shown that in certain soils this microbe can remain alive for four 

 hundred and four days after being placed on the surface. Again, the 

 bacilli of tuberculosis and other diseases find their way to the soil 

 occasionally, and, though not isolated, the microbe causing diarrhoea 

 is probably normally present in the soil. There are some soils which 

 are free from bacteria. These are the waste places of the earth, where 

 the conditions are such that bacterial life is not possible owing to 

 want of moisture or of organic pabulum. Such places are, for example, 

 the great sandy wastes of the world which do not support life in any 

 form, in fact, where other kinds of life are impossible those places 

 will also be devoid of bacterial life. 



As to the method of examining the bacterial contents of a soil, a 

 very simple method consists in mixing a known quantity (about 

 J gram) of soil with a known quantity (about 50 c.c.) of sterile water, 

 and after thoroughly mixing the soil and the water, to pour about a 

 quarter of a cubic centimetre of the mixture into a Petri-dish con- 

 taining liquefied nutrient agar or gelatine. The bacteria contained in 

 the mixture of soil and water are allowed to grow into colonies, which 

 are counted. Thus, suppose we found 200 colonies on a plate : 

 \ c.c. of mixture of soil and water contains 200 colonies, 



.. 50 c.c. of mixture of soil and water contains 200 x 4 x 50 colonies. 

 Now, 50 c.c. contain J gram of soil, 



.-. J gram of soil contains 200 x 4 x 50 bacteria, 



.-. 1 gram of soil contains 200 x 4 x 50 x 4 = 160,000 bacteria. 



This number will not include the anaerobic bacteria. To ascertain 

 their number the same method may be used, but cultivation must be 

 effected under conditions in which oxygen is excluded. 



