96 BACTERIA AND SOIL FERTILITY 



method, as it gives us an insight into the changes actually produced 

 within the soil- We may for convenience consider them under 

 the following heads: 



1. Ammonifiers^ — Microorganisms which break down protein- 

 aceous material with the liberation of ammonia; nitri£ers, micro- 

 organisms which transform ammonia into nitrates. Of these 

 there are two classes, one which changes ammonia into nitrites, 

 the other which changes nitrites into nitrates. 



2. Denitrifiers — Microorganisms which liberate the nitrogen 

 of nitrites or nitrates as free nitrogen. 



3. Cellulose decomposing bacteria — Microorganisms which 

 break down vegetable tissues with the formation of various acids. 



4.4 Nitrogen-fixing bacteria — Microorganisms which take ni- 

 trogen from the air and build it mto their bodies. There are two 

 classes of these organisms, one which lives free in the soil, the 

 other which lives in conjunction with legumes. 



