400 General Botany 



Animal and plant proteins are a part of the residues (5) left by 

 death of the plants and animals. 



Bacteria now become active agents of disintegration. By 

 hydrolysis, reduction, and oxidation the complex substances of 

 the cells are broken up into simpler compounds. Among these is 

 (6) ammonia. This compound, if liberated in soil in the presence 

 of water and carbon dioxide, forms ammonium carbonate 

 ((NHJ2CO3). 



A second group of bacteria, the nitrifying forms, use the am- 

 monia and oxidize it to a nitrite (— NO2). The nitrites are in 

 turn oxidized by other nitrifying bacteria to nitrates (— NO3). 

 The cycle is complete and we are back where the process started. 



Notice, however, that we have only used nitrogen that occurred 

 in organic matter, and that some has been lost by the way, by 

 going into the atmosphere as nitrogen gas. Furthermore, the 

 above cycle is not the only possibility. The soil may not be 

 well drained and aerated, and the (i) nitrates are then attacked 

 by denitrifying bacteria and broken down to (8) nitrogen gas. 

 Thus it is released and becomes unavailable for the higher green 

 plants. So if there were not some means of securing an additional 

 nitrogen supply, the land would become poorer and poorer as 

 time went on. 



The nitrogen-fixing bacteria provide this additional supply. 

 In well-drained neutral soil the saprophytic varieties that ob- 

 tain energy by oxidizing carbon compounds in the humus fix 

 enough atmospheric nitrogen to form the compounds used in 

 building their own cells. At their death these compounds be- 

 come available to other bacteria and the first cycle is repeated 

 with the addition of nitrates from the air to the soil. The bac- 

 teria that live in the nodules of legumes also build the free ni- 

 trogen of the air into organic compounds, and when the bac- 

 teria die the nitrogen compounds become directly available to 

 the legume plant. If legumes are plowed under, the cycle 

 starts over again with plant residues (Fig. 247) and in a few 

 months it has come around to the nitrates. 



