406 BENEFICIAL ACTIVITIES OF BACTERIA 



of replacing or renewing it. The same is true of sulphur and 

 potassium. 



Bacteria of decay are essential in farming. They bring about 

 the decomposition of complex organic compounds into simpler 

 ones. When this is accompanied by offensive odors, the process 

 is known as putrefaction. The complex proteins and protoplasm 

 of the plant or animal contain nitrogen. When a plant or animal 

 dies, the bacteria of decay attack these nitrogen-containing com- 

 pounds and break them down into simpler substances. One of 

 these is a nitrogenous material, ammonia ; another substance con- 

 tains sulphur. Phosphorus and the other elements present may 

 be liberated, similarly. The decay process also sets free carbon 

 dioxide and water. Nitrogenous wastes of animals are usually 

 changed into ammonia by the action of the bacteria of decay. 



Nitrification. Certain bacteria called the nitrifying bacteria 

 take the ammonia formed by the bacteria of decay and change it 

 into nitrates which the plants can utilize for further protein- 

 making. This process is called nitrification. Ammonia is a gas, 

 and would escape into the atmosphere were it not for the nitrify- 

 ing bacteria. Nitrite bacteria first convert ammonia into nitrites, 

 compounds formed by the union of ammonia with oxygen. Then 

 other organisms cause the nitrites to take up more oxygen and 

 form nitrates which are stable compounds and can dissolve in 

 the soil water. Nitrite and nitrate bacteria are always found 

 together. They work best in alkaline soils. If a sample of soil 

 is heated and thus sterilized, the action does not take place. 

 This indicates clearly that the process is a bacterial one and not 

 simply a chemical one. Nitrification takes place only in soil 

 that is well aerated and well drained. The drainage carries off the 

 acids that might interfere with the process. Aeration is necessary 

 because there must be enough oxygen to combine with the ammonia 

 to form the nitrites and the nitrates. Earth worms aid in this 

 process by constantly working through the soil. This permits the 



