DECOMPOSITION OF ORGANIC MATTER WW 



fats, and proteins of animal and plant origin. Those 

 compounds which are insoluble are rendered soluble by the 

 excretion of enzymes which acts hydrolvticallv, usually to 

 dissolve the compound. The soluble substance then diffuses 

 into the bacterial cell where further transformations takeplace. 



Since enzymes act independently of the living cell which 

 produces them, they frequently form sufficient material to 

 kill, or at least to stop the activities of the bacteria as well 

 as of themselves. This is true of acid- and alcohol-forming 

 enzymes in particular. If the acid can be neutralized as 

 fast as it is formed, or if the alcohol is further changed, 

 the production of acid and alcohol will not cease. 



Some bacteria use oxygen, while some do not. This fact 

 goes one step further in that many of the oxygen-using, or 

 aerobic bacteria, can not live at all in the absence of oxygen 

 or air, and in that many of those which do not use oxygen, or 

 anaerobic bacteria, cannot live in the presence of oxygen 

 or air. 



120. Decomposition of Organic Matter. — In the soil, as 

 was stated above, are very many bacteria of all kinds. 

 Furthermore, the conditions under which thev live are very 

 different, depending on the physical condition of the soil 

 and on the kind of organic matter from which they derive 

 nourishment. The principal difference, however, is the 

 presence or absence of air. In a soil that is fairly open and 

 aerated there are different products formed from those in a 

 water-logged or non-aerated soil. It is not easy, however, 

 to classify organic decomposition on this basis, for soils 

 vary gradually all the way from those containing no air at 

 all, like swampy lands under water, to those which are very 

 thoroughly penetrated by the air, like loose, sandy lands. 



In a general way the products of decomposition can 

 be classified into two groups: First, those developed under 

 aerobic conditions; and second, those developed under 

 anaerobic conditions. Under aerobic conditions there will 

 be produced large quantities of carbon dioxide and water, 

 mineral salts (set free by oxidation of organic matter con- 

 taining inorganic elements), and nitrates, but not much 

 humus (Section 122). Under anaerobic conditions there 



