130 LIVING ORGANISMS OF THE SOIL [chap- 



and even insects, worms, and other lowly creatures help 

 to produce the same change. The other change, which 

 takes place in the absence of air, is seen when a branch 

 of a tree falls into a pond or a deep ditch and becomes 

 buried in the mud. Thus cut off from the air it changes 

 much more slowly, but it does lose carbonic acid, marsh- 

 gas, and sometimes hydrogen. Thereby it also becomes 

 gradually black, and on analysis proves to be richer in 

 carbon than the original wood, because it has been 

 giving off in the gases that arise from it more of its 

 hydrogen and oxygen than of its carbon. This process 

 of an anaerobic (out of contact of air) change is seen 

 most markedly in the formation of peat from vegetable 

 matter on waterlogged land, but it has its share in the 

 production of the humus of all cultivated soils. 



We have now reviewed the main types of change 

 brought about by bacteria in the organic matter of the 

 soil ; the carbon compounds are being broken down into 

 simpler bodies and in nearly all cases eventually become 

 carbon dioxide, the regeneration of which into the 

 complex organic matter of the carbohydrates, etc., is 

 effected by the living plant. The nitrogen compounds 

 are also broken down into successively simpler bodies, 

 finally into ammonia and nitrates, which the living plant 

 can again utilise and reconstruct into the more complex 

 protein forms. Some nitrogen also is always being 

 thrown out of combination, and passed into the air in 

 the comparatively useless state of nitrogen gas. It will 

 be noticed that we have seen no agency at work to 

 reconvert nitrogen gas into any form of combination, 

 nothing in fact to account for the original stock of 

 combined nitrogen in the world. Plants can only make 

 use of nitrogen when it is already combined as ammonia 

 or nitrates; they build up successively more complex 

 bodies from these substances, but not from nitrogen gas 



