10 GENERAL CONSIDERATIONS 



quickly lost, never to return. These bacteria, or 

 germs, as they are often called, constitute the minutest 

 forms of life. They are so small that they are scarcely 

 more than visible under the highest powers of the mi- 

 croscope, but they occur in such numbers, their vital 

 activities are so great, and they multiply so rapidly, 

 that they constitute one of the most powerful factors 

 in the world's economy. They are the agents of de- 

 cay. It is by their aid that refuse animal and vegeta- 

 ble matter of all kinds is rotted down so that it may 

 become incorporated in the soil and so become avail- 

 able for the growth of useful plants. Besides the nu- 

 merous kinds whose combined activities result in this 

 most useful work of decomposition there are many 

 others, some highly beneficial and others noxious, 

 whose activities in the soil strongly influence its qual- 

 ity and fertility. Some attack the ammoniacal com- 

 pounds resulting from putrefaction and convert them 

 into nitrites. Others in turn convert these nitrites 

 into nitrates. It is only in the form of nitrates that 

 our ordinary crops are able to use nitrogen as food, 

 hence we see the immense practical importance of un- 

 derstanding how these salts are formed in the soil and 

 of knowing how these processes may be influenced 

 by different methods of cultivation. Nitrogen stored 

 in the soil in other forms represents potential fertility, 

 but it only becomes actual fertility after it has passed 

 through those complicated changes known collectively 

 as nitrification, which are produced by the combined 

 action of these different livinof orofanisms and which 

 result in the formation of nitrates. These nitrates 

 are very soluble, and they are readily taken up in 



