210 MODIFICATION OF THE SOIL POPULATION 



system of the animals, many bacteria develop and contribute to a 

 change in the nature of the organic matter. So extensive is this 

 bacterial development that the cells of the microorganisms com- 

 pose a considerable part of the total mass of the excreted material. 

 In the case of man, it has been calculated that as much as 20 per 

 cent of the faeces consists of bacterial cells. The same is probably 

 true in the case of other omnivorous and carnivorous animals. 

 With herbivorous animals, especially those feeding on less con- 

 centrated foods, the bacterial mass is smaller in amount, due to the 

 fact that a larger portion of the excreta consists of undigested con- 

 stituents of the roughage. The numbers of bacteria per gram of 

 faeces is calculated in billions. Since such large numbers of bac- 

 teria develop as a result of their feeding on the material that passes 

 through the animal digestive tract, these organisms must play an 

 important part in decomposing various organic substances con- 

 tained in the food. 



The nature of the nitrogenous compounds in animal manure is 

 distinctly different from that of the compounds in plant manures 

 which have been considered previously. As a result of the 

 digestive processes, practically half of the nitrogen is excreted 

 as urea which may be quickly transformed in the soil or in stored 

 manure into compounds which are available to plants. In fact 

 the transformation is so rapid that, unless particular care is taken 

 to preserve this nitrogen, it may soon be lost either by volatiliza- 

 tion or by leaching. 



The decomposition of animal manures is slow in comparison 

 with the transformation of certain other plant residues. This is 

 apparent from the curves of Fig. 78. Decomposition, as measured 

 by the formation of carbon dioxide, indicates that within a period 

 of about two months considerably less material was decomposed 

 in ten tons of manure in soil than in one ton of either oats or clover, 

 these being representative of green manuring crops. During 

 this period of 53 days, 60.8 per cent of the clover was decomposed, 

 49.0 per cent of the oats, and only 4.23 per cent of the animal 

 manure. The slow decomposition of the manure is explained 

 by the chemical composition of the material. As shown in Table 

 50, the manure contains relatively large amounts of certain chem- 

 ical complexes which do not decompose readily. The amount of 

 water-soluble material in the sheep manure is particularly high, 

 since both the sohd and liquid excreta were mixed together. It is 



