3 i6 



PLANT FRIENDS AND PLANT PESTS 



bacteria are found within six inches of the surface. An 

 examination of sandy soil has shown something like 100,000 

 bacteria to the gram ; in the soil of an ordinary garden, 

 1,500,000 to the gram, while in the soil surrounding privies 

 and cesspools, as many as 115,000,000 to the gram may be 

 found. 



Bacteria in the soil break down organic material, such 

 as dead bodies of plants and animals, and help to oxidize it. 



Some nitrifying bacteria 

 act upon ammonia (a 

 product formed in de- 

 caying) and change it 

 to nitrites, while others 

 change these nitrites to 

 nitrates, which can be 

 used by plants. Still 

 other bacteria, as we 

 have seen, are found liv- 

 ing upon the roots of 

 certain plants, such as 



The nitrogen cycle. Follow the arrows and find clovers, peas, and beans, 

 out what becomes of the nitrogen. . .. 



Sometimes a good farmer 



buys these bacteria and puts them into the soil, or inocu- 

 lates his soil. These bacteria, although they live at the 

 expense of the plants to which they attach themselves in 

 little nodules, yet take the nitrogen out of the air which 

 is held in the soil, and change it into a form that can be 

 used by the plants. In the end the bacteria have reduced 

 nitrogenous substances to a soluble form which can be 

 absorbed through the roots-. There are many other kinds 

 of bacteria present in the soil, some useful, and some 

 harmful. The denitrifying bacteria which are commonly 



