THE SOIL. 61 



a fairly even temperature, a supply of air and water and 

 no sunlight. Below the water table few bacteria can exist 

 on account of the insufficiency of air. In a soil there has 

 been found : 



micro-organisms. 



In 1 <r\\\m r.f soil at 20 cm*, below the surface (550,000 

 100 ., ., 30,000 



140 .. 700 



;u id going deeper still micro-organisms cease to exist. It 

 can easily be understood that, in spite of their minuteness, 

 bacteria, occurring in such large numbers, must have a 



O O ' 



great effect on the character of the soil. While some 

 bacteria are indispensable to soil fertility others are 

 distinctly hurtful. Some bring about processes of oxidation 

 others reduction : the former can only live in the presence 

 of oxygen and the latter are often of a harmful nature. 

 A few of the most important processes controlled by micro- 

 organisms will be shortly reviewed, attention being directed 

 to the result rather than to the special action of the micro- 

 organisms. 



The " Putrefaction" of organic matter is brought about 

 through the deoxidising, or reducing, action of certain 

 bacteria. These bacteria obtain their oxygen from the 

 combined oxygen in the compounds they attack. It is 

 this class of bacteria which flourishes in the interior of 

 manure heaps and in masses of organic matter, until the 

 breaking up of the mass admits oxygen and allows of the 

 action of other organisms. The products of putrefaction 

 are usually complicated bodies giving off a strong dis- 

 agreeable odour. The "decay "-causing bacteria are the 

 most active in disintegrating organic matter and producing 



