^OIL ORGANISMS 



243 



bacterial and mycelial associations. These find their optimum living 

 conditions in the uppermost soil layer. They diminish rapidly with 

 increasing depth (Fig. 121). Humification therefore is a process 

 chiefly of the surface soil. The decomposition of organic substances is 

 accomplished by putrefaction and decay. 



Putrefaction. — This process goes on best in the absence of oxygen 

 by aid of anaerobic bacteria. Some anaerobic albumin destroyers are 

 Bacillus putrificans, B. paraputn'ficans, Parapledrum foetidum. 

 Bacillus macerans and B. amijlohacter cause the destruction of pectin 



/" V" 8" IZ" 20 



30" 



60" Depth 



Fig. 121. — Bacterial content of garden soils (X and •) ; timothy meadow soils (o); 

 and forest soils (+). {After Waksman.) 



substances (plant mucus) and of the hemicelluloses used as reserve and 

 cell-wall materials. The group which ferments cellulose, decomposing 

 it into fatty acids, methane, and hydrogen, includes, according to 

 Dliggeli (1923, p. 16), B. lyiethanicus B.f ossicular um, and other species. 

 Putrefactive organisms inhabit the upper soil layer, especially where 

 plant remains have accumulated in large quantities, and where the 

 supply of air is small. Especially favorable conditions occur in the 

 mud of swamps and in the drift peat of lakes and large streams. The 

 mineral substances resulting from humus decomposition in mud occur 

 in a form which cannot be assimilated by plants. 



Decay. — Putrefaction and decay go hand in hand and merge into 

 each other. However, while in putrefaction only partial combustion 

 occurs, and the end products are still subject to further oxidation, 

 decay is characterized by almost complete, slow combustion of the 

 organic substances. The main product here is carbonic acid, and this 



