264 AGRICULTURE ON THE PACIFIC SLOPE 



these are called nitrogen-fixing bacteria, because they 

 take nitrogen (a very valuable plant food) from the 

 air and store it in the soil in such form that plants 

 can use it. Some of these nitrogen-fixing bacteria live 

 free in the soil. Others live in the root tubercles 

 (figure 142) of legumes (such as clover, pea, vetch, 

 lupin, or alfalfa). These plants are plowed under (as 

 greenmanure), and by their decay enrich the soil with 

 nitrogen. 



In a compost or manure heap we find some bacteria 

 which cause it to decay and some which make am- 

 monia. Such a heap often smells of ammonia. Other 

 kinds (nitrifying bacteria) change the ammonia into 

 other substances more useful to the plant. They must 

 have air for their growth, and so it would seem best to 

 stir the heap and let in air. But we find other bacteria 

 (the denitrifying) which undo this work and send the 

 nitrogen of the ammonia off into the air. So we should 

 wait until these denitrifying bacteria have ceased their 

 growth. We may then admit air to help the nitri- 

 fying bacteria in their work. This we also do in tilling 

 the soil, and the bacteria greatly help in putting the 

 land into good condition. 



By knowing how these bacteria act, we may help the 

 work of the useful kinds and check the harmful ones 

 simply by watering and stirring the manure heap and 

 the land at the right time (see page 140). 



